My preferred method is: Fill a jar full of undiluted general purpose cleaner like Simple Green or whatever, dropping the miniatures in for a few days, brushing off the loose paint with an electric scrubbing brush, rinse and repeat until satisfied. The electric brush saves so much time, but you'll probably want to make sure to have rechargeable batteries for it.
The most stubborn set of models that I've encountered has been this set of primed plastic bikes I picked up cheap. I don't know what the guy used to prime them, but for a lot of the bikes the primer didn't start coming off until second or third run through the soaking vats.
I know it's really late now because I'm now wondering how effective boiling and freezing metal figures would be to just crack the paint off of a metal figure. Throw the figure in boiling water for a few minutes, then throw it into the freezer. You'd think that would crack the paint off a figure. Too bad I don't have any tongs around to try it out to see if it works well.... :scratchhead: