The trick with stripping old minis, plastic or metal, is to use an alkaline liquid. It should have a pH value of 12 – 14. Many products come with the pH value already printed on the label, so you know if you’re good or not. The best products I’ve found for this are
1. Graffiti paint removers (probably the best)
2. Oven cleaners
Don’t bother searching for a specific brand; they should all work the same if they fall into this category. The best way to do it is to pour the liquid in a glass jar, close the lid to avoid evaporation (and smell), dip the mini and wait around 15 – 20 mins or so. After, just brush it with a toothbrush. If not happy with what you have, repeat until all the paint has gone. You might wanna do it a couple of times to get rid of all paint traces.
They work for acrylic and enamel, although acrylics tend to be easier to remove. The only thing that takes some time more is metallics. They also don’t affect plastic as organic solvents do, so be relaxed.
Also it is a good idea to wear gloves, as high alkalinity irritates skin.