Forward HP is his Crush Counter. Forward MP is a fast overhead. Back MK is pretty useful. Forward Light Kick is the jump forward knee attack that I can't recall the name of.
V-Skill is a good single hit attack and actually safe on block, the large amount of active frames make it a GREAT meaty, I believe it also has slightly longer range than all of your normals, and while it has start up this makes it an amazing poke for people just outside your range, doesn't cancel into anything but can be cancelled into from c.MP. Do it earlier than you think if you're trying to catch a fireball.
Tragedy assault can punish fireballs on read. Tragedy Assault removes meter from the opponent, the EX version also gives you a bit of life. Only the fierce version is an overhead, but it has very long start up, can be decent in neutral or as a punish for a read. s.MP combos into the EX version, the first EX Sonic Boom (only throw one) also combos into the EX version if you time and distance it right.
His Critical Art is projectile invincible, it's very easy to hit Nash out of it with any sort of low or just block if you threw it out at the wrong time, punishing fireballs, jumps, etc. requires the timing you'd expect. You can combo into it, do light or medium scythe kick from whatever you were doing and use that to buffer this for great damage.
Moonsault Slash, the light is safe on block, the medium was at some point don't know if that's still true, it's a getting in tool, though don't do it predictably as they don't need to block if they can just AA you. When fighting Birdie throw booms at full screen to bait his jump grab, use medium Moonsault to counter it, works at least once every game.
V-Trigger is very valuable for extending combos, mixing up with Sonic Boom, full screen punish or getting you out of a tight spot. V-Reversal is an okay version of this, but I'd rather have the Trigger.
Combos, stuff starting with lights (that isn't a target combo such as s.LK > s. MK and s.LP > s.Mp) may no longer work, haven't tested. Any combo ending with Light or Medium Sonic Scythe links into Judgement Saber, I can not stress this enough.
(When combing with EX Sonic Boom, only throw the first one.)
With V-Trigger
j.HK > c.HP > b.MK > Light Sonic Boom > c.V-Trigger > j.HP > c.HP > b.MK > Heavy Sonic Scythe
With V-Trigger and CA
j.HK > c.HP > b.MK > Light Sonic Boom > c.V-Trigger > j.HP > c.HP > b.MK > Light Sonic Scythe > Judgment Saber
In corner with 3 bars and V-Trigger
j.HK > s.HP xx EX Sonic Boom> s.LP > s.MP xx EX Sonic Boom > EX Tragedy Assault > Back V-Trigger >Hard Sonic Scythe
Target to Target link
s.LK > s.MK > s.LP > s.MP Can link into light and medium scythe kick which in turn buffers super
Midscreen
c.LP > c.LP xx Light Sonic Scythe
c.LP > s.LK >c.MP xx Medium Sonic Scythe
c.MP > b.MK > xx Medium Sonic Scythe
c.HP > b.MK xx Medium Sonic Scythe
c.HP > b.MK xx Light Sonic Boom xx c.V-Trigger > j,HP > c.HP > b.MK xx Medium Sonic Scythe
Anti-Air
EX Sonic Scythe xx c.V-Trigger > j.HK > EX Sonic Scythe
Corner
s.HP xx EX Sonic Boom > s.LP > s.MP xx MK Sonic Scythe
s.HP xx EX Sonic Boom > s.LP > s.MP xx EX Sonic Boom > c.HK
s.HP xx EX Sonic Boom > s.LP > s.MP xx EX Sonic Boom > EX Tragedy Assault xx b.V-Trigger >j.HK > HK Sonic Scythe
Extra
EX Tragedy Assault > c.V-Trigger > j.HK > EX Sonic Scythe. For style, does decent damage as well, gotta practice you're timing for this one, remember medium punch combos into EX Tragedy Assault.
s.LK > c.MP xx Medium Sonic Scythe xx Judgment Saber was very useful during early beta tests, not sure if the light kick confirms into c.MP anymore, but you can probably still just start with c.MP.
Move list
Sonic Boom QCF+P
Tragedy Assault SRK+P
Moonsault Slash QCF+K
Sonic Scythe QCB+K
Judgment Saber QCBQCB+P