well i have never gotten a character to 50 yet but i do have a 46 nin/poi MM so here goes.
nin/poi is great at taking down single targets quickly. so many debuffs from your poi set. but at 27 Neuro breath is the only aoe power you really have. so my build went with the leadership PP. with assault and tactics i would try to take down larger groups more quickly with stronger attacks and less misses.
also if you slot your smoke bomb with enough -rech you can either get it perma or almost perma i forget. i would cast smoke bomb on my oni as often as possible. once you hit 32 and get your 2nd upgrade oni with smoke bomb will kill like crazy. smoke bomb oni casting fire breath for instant crit on a mob. beautiful.
ninjas can be very buggy sometimes and need constant babysitting. so you gotta have good macros or binds to reign them in sometimes.
hope this helps
The biggest problem with Ninja/Poison is you picked two sets who each excel at smaller, tougher encounters. The Ninja AOE gets better post 32, but it is middle of the road at best, and that's when you actively use smoke flash and everything goes well.
The plus side is that the combo handles bosses and higher level foes very well and does it without heavy reliance on the Tier 3. For Ninjas, the Jounin are really the force behind the set, with Oni providing some damage and a bit of control.
My advice is contrary to what most ATs will do in the i16 age.......crank the difficulty and take on smaller groups w/bosses rather than larger groups of lower enemies without bosses. Groups get easier post 32 and again post 38, but your strength is still taking out hard targets with great damage and massive debuffs.
at 38 noxious gas is a real EB killer. been a while since i played but i think it'll do very well on AVs as well. unfortunately the oni tends to be more of a ranged attacker and only occasionally gets in close to attack. noxious gas is more powerful on your tier 3 but jounins are more likely to get in close at a quicker pace. unless you really get a handle on controlling your pets noxious gas is better on a jounin.
Where the Oni attacks from is user error........making the error of letting him decide in the first place. 
/poison is great, just misunderstood when compared to other secondaries. Just remember to keep the boys in check and spam the hell outta your debuffs and you should be fine. I respec'd out of toxic breath, but all the other moves are pretty handy.
I'm playing a Ninja/Poison MM at the moment. Im up to level 29 and having fun in that kind of mad seat-of-your pants kind of way.
Poison does really puzzle me as a set. I've got all the powers that I'll be taking now, except Noxious Gas, and I still dont see what the set does so well.
The heal is a standard sub-par patch up like Storm's. Fair enough.
The rez and Clear Mind clone are pretty standard.
It has a reputation as a single target debuffer, but to be honest, Weaken and Envenom are really not all that great.
Envenom is pretty much eclipsed by Freezing Rain or Acid Mortar, both of which give comparable -Res and -Def debuffs to a whole group at once, and are still up every fight once slotted. Even Acid Arrow looks a bit better - smaller values but the small AoE lets you debuff multiple targets.
Weaken provides -11.25% To Hit on a single target, same as Darkest Night OR Fearsome Stare, both of which are AoE. Hurricane gets -22.5% to Hit, on multiple targets. I definitley feel unsafe with this set due to low-ish To Hit debuff and poor heals.
OK, Weaken and Envenom do stack with Noxius Gas, but thats on a long recharge.
Now I understand I'm playing one of the squishier primaries here, along with a set that doesnt offer much passive defence. And I havent started to really hit EB-ville by level 29, where the ability to take down single hard targets may shine.
What am I missing? Does the -Special in Weaken really shine later on? Is Noxious Gas that good with its short-lived Mag 1000 hold?
Hey all,
I have 3 MMs I enjoyed to 50, Nec\Drk, Thug\FF and Merc\Traps.
In this gap waiting for Going Rogue I have started to level a Nin\Pois to fill in the nights where the VG is not on. No better solo character than a MM.
I have read the guides and I have a good handle on my general build structure, but at level 27 I have found battles with larger groups dicey to say the least.
I know how life changes getting the third tier pet and the second upgrade, so maybe I need to park the respec for later.
My question to everyone is what pool powers have you mixed in to add playability and fun? I have seen stealth and invisibility mixed in, I have also seen posts where Provoke/Challenge works well.
I have not had a taunt power on a MM before, is it just a matter of being in BG mode and cycling it over and over? I was thinking whirlwind as a concept like Tsoo sorcs since that is another power I have never taken.
Appreciate any tips, tricks or advice...
Murph
Murph's 50s:
H: Tank, Cont, Def, Scrapper, Blaster, Bane
V: MM x5, Corr x2, Stalker, Dom, Fort