1 Meter: HP double > light Big Bad Boot > HP double > light Battering Ram > Shadow Battering Ram > Big Bad Boot ender (52%)
2 Meter: HP double > light Big Bad Boot > HP double > light Battering Ram > Shadow Battering Ram > Shadow Big Bad Boot ender (61%)
( Damage calculation notes)
Notes: Rash would like to use Big Bad Boot linkers during the combo, but they take too long compared to the lower damage Battering Ram linkers, so he can only squeeze one of them in at the start of the metered combos. Rash can't get to level 4 if he counter breaks without any white life on his opponent, so the 1-meter combo needs to throw in a shadow linker in order to boost that damage as listed. In real matches, though, you will tend to prefer duplicating the 0-meter combo and sticking on a shadow Big Bad Boot ender, which boosts the damage more than shadow Battering Ram does.
Rash's only traditional grounded special move opener is Battering Ram, a horizontal charge that is both fast and long range. The light version is the safest at -2, meaning Rash gives up his turn whenever this move is blocked. It is a decent move to use in pressure, especially if you expect a move like Rash's crouching MK to hit, but a smart opponent will challenge Rash after it is blocked. The other strengths of Battering Ram are more negative; medium is -4 and still cannot be punished by normals, but heavy is -10 and will get Rash killed. On the other hand, both medium and heavy Battering Ram are fantastic whiff punishers, traveling two-thirds of the screen in an instant. As the medium version cannot be punished by normals, it is an excellent mid-range "check" move to keep your opponent's attempts to advance honest.
Battlemaniac is a target combo that Rash can access by pressing either LP or LK repeatedly. If you continue to press LP or LK, Rash will go through a series of jabs, then a Battering Ram shadow linker, and finally a shadow Big Bat Boot ender. Generally you will not want the entire sequence, however, and will convert the second or third jab in the sequence into a normal combo, as Battlemaniac is special cancelable. Unlike other target combos in the game, the second hit in this sequence is marked as an opener, which means a special cancel will immediately go into the linker version and the third hit in the chain and beyond becomes breakable under normal rules. Battlemaniac is actually a surprisingly useful close-range tool for Rash; the light attacks are good at keeping an opponent in place, Rash can stop and mix in a throw or Wicked Tongue mixup at any time, and if the opponent tries to jump away from Rash's pressure, Battelmaniac's third hit will automatically recapture them.
Aside from his standard grounded openers, Rash has several other ways to set up combos.
Wicked Tongue is a zipline move that can be steered in any of the 8 directions and optionally used in the air. It is perhaps Rash's most important move; it swallows nearby projectiles for large shadow meter gain, and it is his main method for mobility, often flustering opponents who are not ready to handle its speed and versatility. If Rash's tongue hits a grounded opponent, Rash will have enough frame advantage to manual a light attack and then start a combo. If he makes contact with an air opponent, or if Rash uses the move from sufficiently high in the air towards a grounded target, Rash will be able to attack with an air normal directly after. Because Tongue is a special move, it can also be canceled off grounded normals for very fast surprise approaches. Do note, however, that it does 0 damage; starting a combo with Tongue will not hurt very much, since you will not get any first hit damage bonus.
The most important thing to know is that, when used close to the ground, Wicked Tongue is -5 on block, which is punishable by light attacks. Because the move happens very fast, however, it is easy to let this window pass and allow Rash to get away with offense that is not tight or safe. When playing against Rash, try to specifically watch for Tongue and press light attacks as soon as you see yourself blocking the move. Even if you don't successfully punish, challenging Rash's next move is really paramount, or else he will just use Tongue repeatedly and run you over.
Big Bad Boot, a gigantic kick of Rash's comically oversized foot, will always launch the opponent. Rash can optionally cancel Boot into his forward run on hit or block, but importantly, Rash can continue the combo for a juggle if he launches with either medium or heavy Boot. The light is the safest on block at -2, but will not cause a high enough juggle for Rash to pick up the combo if he uses it first. Rash can continue the higher Boot juggles in any number of ways, including canceling into run and then pressing a far ranging normal like standing HK and canceling into more Boots or Wrecking Balls. In the corner, you can simply juggle Boots back to back with varying strengths, as long as the KV allows! The higher strength Boots that cause bigger juggles are also less safe on block, though, with both medium and heavy being punishable by normals if done very close. However, if Boot is used in the mid-range, perhaps canceled from standing HK, it will often hit on its later active frames and be deceptively safe.
Wrecking Ball is a move that often frustrates newer players. Rash transforms into a large wrecking ball and flies across the screen in an arc. This move, which can be done on the ground and in the air, has one hit of armor, will cause a wall splat if close enough to the stage boundary, and allows Rash to attack on the way down with any air normal when he's finished. The different strengths have slightly different arcs, meaning Rash will end up near your head at slightly different positions. Importantly, there is almost always a decently large gap between Wrecking Ball finishing and Rash trying to attack from the air, which means you should be looking to anti-air after blocking any Wrecking Ball. While some taller characters might struggle to find a move that always hits at the correct angle, it is important to punish Rash if you block Wrecking Ball and force him to approach using moves that do not have armor. Especially if your character has an uppercut-style anti-air move, this move should be too risky for Rash to use without care.
And last but not least, Rash's grounded overhead, called Rash Bash, will also start a combo. It can be followed up by a manual of any strength with easy timing, but Rash Bash itself will trigger the combo system to start, so your next manual will be breakable and any special moves you cancel into will be linkers or enders. It is a potent move in Rash's arsenal, however, especially considering Rash's strong reset potential, and it should be represented often at close range.
Battering Ram is the faster linker of the two, and as a benefit it carries the opponent towards the corner quickly. Manual timing after Battering Ram linkers happens extremely quickly, so you'll need a bit of practice to be consistent. Big Bat Boot linker is among the slowest linkers in the game, with Rash doing an exaggerated hop kick for each attack. The heavy linker is trivial to break on reaction, but it can be difficult to tell the difference between the medium and the heavy linker on the second hit, so you will often wait for the third hit to break on heavy, only to see the two-hit medium linker slip past you. Big Bat Boot linkers do more damage to make up for their slower pace.
Shadow Linkers: Rash only has one shadow linker, and it is fairly standard in terms of timing and rhythm. The screen freeze is quite short, so be ready to break fast if you are looking to catch the first hit.
With no battery ender to fuel Rash's meter, he can only choose between a damage ender and two different setup enders. Big Bad Boot as a regular ender does pretty decent damage, but the shadow damage ender really shines and it should be how you invest your meter if you are looking to maximize the damage in your combo. It is also extremely useful for cashing out Rash's many juggles, so be sure to save some meter for that. Wrecking Ball is one of the best wall splat enders in the game due to how far Rash pushes his opponent; you can be a great distance away from the wall and somehow the ender will find a way to make the wall splat work anyway. Especially when you don't have meter for shadow Boot, the severe corner push makes this ender a wise choice from almost anywhere beyond half screen. Battering Ram launches the opponent over Rash's head and causes a hard knockdown. If you walk forward a brief step, then pause, then jump forward, Rash can get some tricky crossups going with jumping MK, which can be timed to hit in the front or back, as well as downwards Tongue to disguise the trajectory and keep Rash in front on what looks to be an "obvious" crossup.
His instinct is definitely a little awkward to use at first, since your attributes are not just magically powered up for easy comeback potential. Its main use is for mixup safety and for recapturing airborne opponents with an unbreakable attack. For example, if Rash does shadow Boot as a reversal, on hit he can cancel into instinct and recapture with the bike and continue a grounded combo. If instinct is already active, Rash can manually call the bike to keep combos going as well, such as off Boot juggles or throws; from most positions on the screen, press HP+HK while holding the opponent after throwing them to toss them directly into your bike. You can also apply some tricky setups using the bike. After a Battering Ram ender, try doing Wrecking Ball while the opponent is knocked down, and then call the bike while you are close to traveling over their body. Depending on your timing, the bike will act as an assist that hits crossup sometimes, and in the front other times, and then Rash can pick up the combo.
When not pressing standing HK, you can make use of crouching MK, Rash's best cancelable low attack. A great "shoto"-like harassment low, you might look to cancel it into light Battering Ram for a combo starter. It's best to avoid trying to use Boot after crouching MK, because you will often be too close to make it hit meaty, thus leaving you in a worse position. Standing LP and standing LK (which, when pressed multiple times, will trigger the Battlemaniac target combo) are good low pushback normals to use as harassment up close, and Rash Bash works wonders as a quick combo-starting overhead amidst Rash's fast-paced offense. His notable air normals are an excellent, omnipurpose jumping MK crossup and both jumping HP and HK, which hit a long way below and in front of him, respectively.
Shadow Big Bad Boot is a great reversal due to its exceptionally large hitbox, which is almost impossible to avoid blocking from close range if you are trying to prevent Rash from instinct canceling it. Normal Boot, while not invincible, has tons of active frames and a similarly sized hitbox, and the light version comes out sufficiently fast enough to work as a very strong anti-air. While it is possible to use Boot (and other specials, such as Wrecking Ball) such that they hit later in the active frames and become safer, it is very important to note that all of Rash's special moves are negative on block, which means learning to smartly challenge and take your turn back after Rash commits to a special move is absolutely crucial to making sure Rash doesn't get away with more than he should. As a Rash player, to overcome this, try to use special moves at unpredictable times such that your opponent can't keep up with the pace. On occasion, don't cancel normals like standing HK into any special move to throw off the balance of the fight, and if a special move does happen to get blocked, don't be afraid to force the issue with the invincible shadow Boot, especially if you have a fully loaded instinct on deck.
Because learning juggle paths can be daunting if you are just picking Rash up, my advice would be to focus first on mobility and ground control. Experiment with using the different directions of Tongue to move around the screen, for both real and fake approaches, and how you can force your opponent to block an air normal after using aerial Tongue from an unpredictable range and angle. Press a lot of standing HK and crouching MK when you're faced with an opponent a few character lengths away, and force them to jump into a light or medium Boot anti-air. When you do launch them high with Boot, start with a few easy juggle paths, such as running forward, then canceling a grounded button into Wrecking Ball or up-forward Tongue, then hitting an air normal on the way down (when you land, repeat the procedure). Since spacing and timing will always be slightly different, it takes a bit of practice to fully convert all of Rash's juggle opportunities into damage, so don't worry if you drop more than you land at first. Find the mix of air and ground control that works best for your playstyle, because Rash excels in both.