Put at least two, if not three ACC in your T1 and T2 henches.
For pvp :
Genin - 3 acc, 3 dmg OR 2 acc, 3 dmg, 1 disorient
Jounin - 3 acc, 3 dmg OR 2 acc, 3 dmg, 1 def debuff/confuse
Oni - 2 acc, 3 dmg, 1 hold OR 2 acc, 3 dmg, 1 end red
Smoke flash : it's awesome, i recently got it with my 2nd respec and i love it, i suggest slotting it with end reds and recharge reds so you can perma it ( if your endurance allows it 
).
Also ninja won't do criticals from shadow fall, or grant invis, because they need the "hidden" status like stalkers do, and that is only granted from their Hide and smoke flash.
And you don't need that many slots on Kuji in Zen, usually 2 recharge reds are ok ( since you won't find you'll need to use it too much during fights to justify for the end reds ).
Thanks for the replies, guys! That's helped a lot. 
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Smoke flash : it's awesome, i recently got it with my 2nd respec and i love it, i suggest slotting it with end reds and recharge reds so you can perma it
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The hero builder doesn't allow end reduction enhancements, but there is that bewildering accuracy option there. I'll assume that it's broken and go with recharge/end reds.
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Also ninja won't do criticals from shadow fall, or grant invis, because they need the "hidden" status like stalkers do, and that is only granted from their Hide and smoke flash.
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I thought that might be the case, but never hurts to ask. 
Thanks for clearing that up for me!
Hey all! I've managed to snag a respec on my lvl 27 MM (thanks to all who were on the team for that!), and I need some help with the build. Here's the respec build I have so far:
Have at!
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Name: Hand of the Beast
Level: 41
Archetype: Mastermind
Primary: Ninjas
Secondary: Dark Miasma
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01) --> Call Genin==> Acc(1) Dmg(3) Dmg(3) Dmg(5) DisDur(5) DisDur(7)
01) --> Twilight Grasp==> Acc(1) Heal(7) Heal(9) Heal(9) Rechg(11) Rechg(11)
02) --> Aimed Shot==> Acc(2)
04) --> Tar Patch==> Rechg(4) Rechg(13) Rechg(13)
06) --> Train Ninjas==> EndRdx(6)
08) --> Teleport Foe==> Acc(8)
10) --> Darkest Night==> EndRdx(10) EndRdx(15) TH_DeBuf(15) TH_DeBuf(17) TH_DeBuf(17)
12) --> Call Jounin==> Acc(12) Dmg(19) Dmg(19) Dmg(21) Empty(21) Empty(23)
14) --> Teleport==> EndRdx(14)
16) --> Assault==> EndRdx(16)
18) --> Tactics==> EndRdx(18) TH_Buf(25) TH_Buf(25) TH_Buf(27)
20) --> Shadow Fall==> EndRdx(20) EndRdx(23) DmgRes(37) DmgRes(40)
22) --> Smoke Flash==> Acc(22)
24) --> Fearsome Stare==> Acc(24) Fear(27) Fear(29) TH_DeBuf(29) Rechg(31) Rechg(37)
26) --> Oni==> Acc(26) Dmg(31) Dmg(31) Dmg(33) Hold(33) Immob(33)
28) --> Petrifying Gaze==> Acc(28) Hold(34) Hold(34) Rechg(34)
30) --> Recall Friend==> EndRdx(30)
32) --> Kuji In Zen==> Rechg(32) Rechg(36) Rechg(36) EndRdx(36) EndRdx(37)
35) --> Hover==> Fly(35)
38) --> Dark Servant==> Acc(38) TH_DeBuf(39) TH_DeBuf(39) TH_DeBuf(39) Heal(40) Hold(40)
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The Dark side I'm fine with; it's the Ninja bits that I have questions on. Sorry if I'm repeating any oft-asked questions, but I couldn't find the answers on a trawl through the MM board.
I'm really not sure how best to slot the boys. My best guesses are up there (along with a couple of cluelessly empty spaces), but I'd really appreciate some pointers for this. What's worth slotting for, and what's usually a waste of time?
Smoke Flash is something I hardly use at the moment, and I really need to play with it more. Why does the hero builder give me the option to slot it for accuracy? I assume this is for the placate effect? Also, does Shadow Fall give similar critical hits from stealth? (I don't have it in my current build, so I haven't had a chance to test this!)
Any help/comments gratefully received.