Quite a number of evasive tactics you have. Hmm spot removal, yeah (Go for the Throat), so you can get rid of their critters when Mistblade and Slitter can't get through , or a one cmc discard spell (Inquisition of Kozilek/Duress/ Despise/Thoughtseize), or the new Sai of the Shinobi (to raise your ninja's power level as they enter the battlefield). ^^
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Something better than stabbing pain, perhaps. Invisible stalker (sub for Dimir Infiltrator) and Wingcrafter for evasion. Maybe Ornithopter so you can cast it for free after bouncing, seems really cost efficient.
Not sure if 20 lands will support the number of large creatures in your deck. Invisible stalker might be good and also Ornithopter. No need to bounce your ninjas that much if you can effectively give them evasion (whereas Wingcrafters will do). Maybe only one Silent-blade Oni will do.