G'day Chris,
Mr Muscle works really well as Marc said. You have to be careful with soft, hollow plastic figures like Historex as sometimes the chemical reaction caused by the oven cleaner can eat into the plastic itself.
Rather than let the oven cleaner stay on for too long and run the risk of damaging the plastic I suggest you wash off the oven cleaner after an hour or two, scrub the figure with an old toothbrush to remove any loose bits and then spray Mr Muscle on again.
Its safer to do this process over a few times than try to do it in one go, and find later that you've left it on for too long and your figure is ruined.
I have used caustic soda to remove enamel paint from resin figures, but its quite dangerous to use and you need protective gloves and breathing gear if you're doing it indoors.
Hope this helps mate.
Cheers