1 Meter: HK double > light Immortal Spirit > HK double > Shadow Skull Splitter > Conqueror ender (59%)
2 Meter: HK double > light Immortal Spirit > HK double > Shadow Skull Splitter > Shadow Conqueror ender (64%)
( Damage calculation notesThese damage numbers were achieved after light Skull Splitter opener, followed by a counter breaker. Keeping the initial combo as small as possible helps isolate the counter breaker damage, but this is an unlikely scenario in real matches. You can expect your damage to be much higher if you counter break in the middle of a normal combo.)
Notes: Tusk's HP auto-double is much slower than his HK auto-double, so these combos must use HK. Immortal Spirit is both a faster linker and a higher damaging linker than Skull Splitter, so it is the clear choice for Tusk's damage loop. Shadow Skull Splitter is where Tusk gets a lot of his damage, but it is so slow that there is not much room for anything else. These combos are designed so that the last two hits of shadow Skull Splitter hang into the breakable window, which keeps them unbreakable but allows for the most possible damage beforehand.
Because deflect windows are short and not active directly on startup for most moves, deflecting an opponent is difficult to do intentionally and is the type of thing that occurs occasionally as a side benefit for pressing Tusk's best buttons in neutral. The primary goal of deflect is to make your opponent nervous to press buttons in Tusk's face, because he does not want to get deflected into a big combo. When the opponent becomes hesitant, this gives Tusk more liberty to use his other tools, such as Immortal Spirit or regular forward dashes to close the distance and apply close-range mixups.
Deflect is just one of the reasons why Tusk is a strong bully character in Killer Instinct. His large mid-screen buttons have not only large, threatening hitboxes with easy hit confirms when they land, but also the possibility that you can't interrupt the startup because you could be deflected.
+HP to perform Skewer, which makes Tusk jump into the air and slam down on his opponent. These knockdowns include both of his throws, his sweep, jumping HP, and his hard knockdown ender. Skewer typically adds around 4-5% to any combo and adds a pretty healthy chunk of white life, so it is often worth going for.
However, Skewer is always combo breakable with heavies, and it is very easy to see coming. It is extremely important to note that you will still take the Skewer damage if you break, but you will heal half the white life that Skewer does and the combo breaker may send Tusk to a range where he doesn't want to be. Skewer is also counter breakable, just like all breakable attacks, if you want to get particularly frisky. You should likely follow up most of your knockdowns with Skewer just for the added damage, especially in matchups where you don't mind being placed full screen if your opponent breaks. Then, accept some breaks and try to counter break at an unpredictable time; Tusk's massive damage output will do the rest.
Skull Splitter is going to be your primary offensive special move. It is a safe on block move that covers massive screen real estate and leaves good distance between you and your opponent on block, giving Tusk the space he needs to operate. Medium and heavy Skull Splitter come equipped with Deflect windows so challenging them is risky, while light Skull Splitter looks a lot like Tusk's standing MP normal; all Skull Splitters cause sparks to fly when the sword hits the ground, which is how you can tell the difference. Canceling into Skull Splitter after fast long-range pokes like standing MK is great for harassing opponents.
Immortal Spirit's three followups, Moosejaw, Collateral Damage, and Destroyer, are great for starting combos after Tusk has moved through a fireball. While he technically has low (Collateral Damage) and overhead (Destroyer) options after Immortal Spirit, in practice they are difficult to use as mixups, because the overhead is much slower than the low attack. However, while Destroyer is slow, it is notable for its ability to cover the entire screen. Especially if used after light Immortal Spirit, Tusk can space the attack from nearly full screen away and still be safe from most counter attacks. Destroyer is an opener so it moves into the combo system, but it also causes a stagger, so you can easily hit confirm after it with any attack of your choice.
Tusk is a stagger monster. Standing MP and HP are very slow, massive buttons that cause chip damage on block. They are also notable because, outside of instinct, they cannot be special canceled. However, because they cause stagger, they leave you with tremendous frame advantage, which lets you connect an follow-up attack of your choice. Popular choices include light Skull Splitter, the two relatively fast Immortal Spirit follow-ups (Moosejaw or Collateral Damage), and both shadow Skull Splitter and shadow Immortal Spirit. Shadow Skull Splitter is largely preferred, and it works from surprisingly far away, at all ranges except the extreme tip of standing MP range. If you are scared it will be out of range, you can sacrifice some damage and default to shadow Immortal Spirit, which works from slightly farther out. Note that all of Tusk's stagger normals will trigger the combo system to start, so all follow-up attacks will be combo breakable.
If you perform Back Stab from far away, it will not cross up and knock down, but instead cause a stagger. This version of Back Stab has the highest single-hit damage in the entire game at around 22%, but hitting with it is difficult. If you do manage to hit with it, immediately combo shadow Skull Splitter after the stagger. Even if your opponent breaks the shadow move, your damage will still be very high, and not breaking the shadow move will put your damage in the range of 50%.
Tusk only has two linkers. Skull Splitter hits 2, 3, and 4 times for light, medium, and heavy, with similar animation style to Thunder's Triplax linker. Mixed in with MP and HP auto-doubles, which both swing his sword, Tusk can make pieces of his combo difficult to tell apart. Immortal Spirit linkers use the Moosejaw shoulder charge animation, but also are much faster than Skull Splitter linkers and do more damage, so they will be preferred any time you are not trying to cause lockouts.
Because Tusk's standing MP and HP normals are uncancelable and very slow, you should never use them as manuals. Instead, always use standing MK and HK when trying for medium or heavy manuals. Shadow Skull Splitter is a very high damage shadow linker, but because it is so incredibly long, you will have to throw it immediately into combos when you see a lockout to avoid giving the opponent a chance to break the final 3 hits. If you need to throw some extra damage in quickly, shadow Immortal Spirit linker is your better bet.
Conqueror is a fairly beefy damage ender, befitting Tusk's nature as a huge bruiser character, so it is recommended especially to close out rounds. It also leaves Tusk pretty close, which gives him an opportunity to apply meaty pressure. The other enders shouldn't be ignored, however. Skull Splitter functions as a hard knockdown ender, which gives Tusk a chance to use Skewer or set up some other planned situation. Immortal Spirit is a unique ender because it gives Tusk a free stagger; it is, effectively, a mid-screen wall splat ender, because being staggered follows identical rules to being in a wall splat state. Especially when you finish combos with a low ender level, using the stagger ender will give Tusk a chance to set up highly damaging throw mixups (neutral jump with jumping HP for extremely high damage if you predict a throw), and most of his post-stagger mixups will knock down, giving him a chance to use Skewer for even more damage.
Tusk gains two primary advantages with his instinct mode. He can cancel standing MP and HP into specials, which are normally uncancelable buttons. This gives him even better space control, as he can, for example, cancel standing MP into Immortal Spirit and surprise with a throw. Or, you could cancel these buttons into light Skull Splitter for slightly better frame advantage on block. Because your opponent can be conditioned to respect Tusk's giant buttons due to the Deflect windows, these surprise moves can be very effective.
The other main benefit is that he can cancel special moves into special moves, at the cost of a small chunk of his remaining instinct gauge. This gives Tusk a chance to go berserk by, for example, canceling attacking special moves into movement (Immortal Spirit) and then back into attacking moves. If you use medium Skull Splitter (a Deflect move) as your attacking move, you can move across the screen without attacking while Tusk has regular Deflect windows, which could catch an opponent as he presses a button at the oncoming train. He can also cancel his light Conqueror uppercut into Skull Splitter (both the grounded or air versions), and then back into Conqueror again.
You can also do unlimited block strings by doing, for example, light Skull Splitter into Immortal Spirit > Slide, looping these two moves repeatedly until you run out of instinct. At any time, you could cancel into the Immortal Spirit overhead, do empty Immortal Spirit into throw or forward+MP crossup, or any number of tricks involving delayed attacks to catch people flinching. You can add to the chip damage by doing shadow Skull Splitter. And the best part of all is that none of this is shadow counterable. If someone tries, input a heavy Conqueror DP on reaction to the shadow counter screen freeze, which has a frame 1 Deflect window and will catch the shadow counter. Then, after the screen freeze ends, cancel the heavy DP as it's deflecting into heavy or shadow Skull Splitter, which starts a high damage combo. Your only option on defense is to sit there and try to block it out.
+MP command normal when performed close, crosses up and knocks down. Though unsafe if blocked, it is difficult to predict in Tusk's pressure, and you need to represent this normal a few times per round so that the rest of Tusk's offense gains credibility. Spine Splitter, his 
+HP command normal, can be held and dash-canceled at any time. Tusk's forward dash is quite fast for his character archetype, so if you mix in a dash cancel and throw alongside your stagger normals and Skull Splitters during pressure, it will be very difficult to defend.
For jumping normals, jumping MK is Tusk's go-to crossup. Jumping HP is incredibly slow and very unsafe on block, but it does extreme damage and causes a knockdown which leads to Skewer if you choose; jumping HP with a Skewer follow-up does 21% damage. It's not a normal you will use in pressure, but it's great to use as a punish for throw whiffs. Perhaps his best jumping normal is jumping MP, a long-range Glacius-like sword attack which covers huge screen space at an angle in front of Tusk. Use it frequently to irritate people who try to get out of your standing MP range.
From far range, Tusk can struggle against zoners at times. To get around projectiles that cover the area near Tusk's feet, such as Glacius's Hail, he can't easily use regular Immortal Spirit. Instead, use shadow Immortal Spirit, which is fully projectile invulnerable for the duration of the dash, as well as the duration of the shoulder attack that occurs when you press punch a second time. Another approach is to use the air version of Skull Splitter. It will cause a small shockwave that can nullify projectiles or otherwise keep an opponent preoccupied, and when Tusk lands, he has the option to cancel into a shadow move such as shadow Immortal Spirit to close the gap while being shielded by the shockwave. This can be an expensive approach if it doesn't lead to a hit, but for extremely pesky zoning characters, it's an option you may need to represent at times.
But when Tusk is on a roll, he is among the more satisfying characters in Killer Instinct to play. Varying the timing of your button presses and being unpredictable with Immortal Spirit and dash-canceled Spine Splitter command normal will make opponents feel jailed and helpless, and when Tusk lands a hit, he makes it hurt. Being on point with your anti-airs is extra crucial for Tusk, because opponents will prefer to jump to try and avoid his stagger normals and Skull Splitter once they learn that a grounded approach is risky. To beat Tusk, opponents will have to respect him at the mid-range just the right amount; overstepping your bounds or being too tentative will give Tusk a huge edge in the fight.