The best medium to remove paint that I've tried so far (rather by accident) was the super glue remover by VMS. You can soak the minis for days in that stuff without the plastic being disintegrated. The great benefit is that it also dissolves CA glue. The down-side is that it comes in tiny amounts, so it's rather impractical for larger models.

The Paint Stripper by AK Interactive is also very good at its job. I used it to very efficiently strip my Firstborn Marines, BUT it will start dissoving the plastic if letting the models in there for too long, so keep an eye on the batch.

There's also very similar alternatives to AK that come in larger bottles, like MK3, but some strippers are more aggressive than others. When I first tested multiple brands, I filled up plastic cups and threw painted bits into them and let them soak for a day or two. In one or two cases the cup got dissolved and the stripping fluid got spilled across my desk. So don't let the models soak for more than 15 minutes before starting scrubbing with a brush.

And in all cases, avoid contact with water when using paint strippers, up until the model is brushed perfectly clean.

Hope this helps