Turn 1: Toxic (15/16)
Turn 2: Fire Spin (12/16)
Turn 3: Taunt (7/15)
Turn 4: Flamethrower (Dead)
Granted, this leaves a lot up to chance like Fire Spin hitting and lasting more than 2 turns, them not having T wave, and them not switching out right after the Toxic, but it does work if a trapper is what you need. It works on more than just Chansey too.
Also, assuming they Seismic Toss all 4 turns, you'll die too, so you might want to pair it with a ghost type that can switch in on turn 4 and finish Chansey off. No matter what, you'll be pretty much dead, but that was kinda the point of Dusknoir, so I guess that's kinda mute.