Anti airs:

cr. MP - Probably his most consistent option. Hurtbox shrinks and hitbox is directly overhead, but not very wide. Can be cancelled to HP fireball for a little chip if you like.
st. MK - Punishes jumps that are further out than what cr. MP can reliably hit. St. HK works in the same contexts for a bit more damage but doesn't go as far up or out and doesn't come out as fast. Consistency versus damage.
air throw (or jump back MK/LP/LK) - Good for anti airing close jumps. Knocks down, no invincibility but high surprise factor and a mixup to anticipate. 
EX sonic scythe - Probably his strongest anti air and has at least some invincibility but costs bar.

You can anti air super too but you've gotta be really fast and doesn't work from super close so it's not great.


Punish combos:

cr. HP, st. MP xx HK spin scythe (230 damage, 400 stun) - This only works point blank, so from afar swap the initial link with just a st. HP

Nash doesn't seem to have any 1 bar combos to significantly increase his damage

cr. HP, st. MP xx MK SS xx CA (450, 320)


Punish combos USING V-trigger:

st. HP xx SB xx f+trigger, j.HP, cr. HP, st. MP xx HK SS (314, 565)

Nash's super combos do similar damage regardless of trigger, so use trigger or super in a combo but not both.


Crush counter punishes:

st. HP xx LK moonsault, st. LP xx MP xx HK SS (290, 502) - You can also use EX moonsault and go straight to st. MP which does 345, 535 which is OK.
st. HP xx LK moonsault, st. LP xx MP xx MK SS xx CA (448, 442)
st. HP xx EX moonsault, st. MP xx Sonic boom xx f+trigger, j. HP, cr. HP, st. MP xx HK SS (415, 670) 


Ranged Punishes:

cr. HK (90, 150)
EX SS (145, 200)
LK SS xx CA (382, 100)
cr. HP xx trigger, cr. HP, st. MP xx HK SS (258, 460)


Confirm combos:

All previously listed link combos function as hit confirms. Here are some others:

cr. MP, st. MP
moonsault, st. LP
EX moonsault, st. MP
cancel to fireball, f+trigger


Confirm combos (light normals):

cr. LK, cr. LP xx LK SS (111, 213) 
cr. LK, cr. LP xx EX SS (163, 293)
cr. LK, cr. LP xx LK SS xx CA (347, 213)
cr. LK, cr. LP xx EX sonic boom x2 xx f+trigger, j. HP, cr. HP, st. MP xx HK SS (256, 488)
st LK xx MK, st. LP xx LK SS (164, 288) - all the same combos apply except the EX boom combo. Loses low but does more damage.


Non CC punish CC confirms:

f+HP, HP tragedy assault
f+HP (dash?), MK SS xx CA
f+HP xx f+trigger, j. HP, cr. HP, st. MP xx HK SS


counter hit BnBs:

f+MK, cr. LP - this is really useful!!!
f+HK, st. LP
cr. HP, st. HP
v-skill, st. LP
cr. LK, st. LP xx MP - for some reason this only works on counter hit or crouching opponent now.
moonsault, st. MP


midair connect and juggles:

n.j HK, LK SS
n.j HK, CA
anti air boom, CA

Other:

You can combo out of EX tragedy assault into v-trigger, but at present it doesn't seem particularly useful