If you don't care much about the paint, you can also let you miniatures sit in a house cleaing product with amoniac (I really recommand using glanzer, the bright green version).
It's mainly used in miniatures to remove old thick layers of paint, but with enough time you can also deglue parts (I lreaned it the hard way haha).
Avoid with finecast, it can damage the miniature.
But with plastic, let it sit a couple of hours, it should do the trick (let it sit all night if it really doesn't want to dissolve).
The longer but safier way is a scalpel (I'd recommend a new blade), and maybe some help with hand drill (usually used for pinning), it can helps, and do < 1mm damage, but can do the trick for heavy glued parts. Cut from different angles to fragilise the joints.
And if you go full scalpel,
remember to make some pauses, avoid working more than 10-15mn straight. Avoid tendinitis