Intrigued, I dusted off my sets and had a look.
Firstly not all northstar sets have this warping issue, the older stock do but newer ones are printed on more solid resin base and don't have this issue.
Second, like most resin, they become more mellow at higher temperature. I dumped mine in a bit of boiling water and 'unbended' the sections what I want to work with (can use either fingers or tweezers or etch folding tools, what not. If the issue is yours is so bent there's little working space, then that should help.)
Third, northstar figures have a bit of a 'block' below the feet that can be cut into. I use an etched saw because, unlike a knife, which 'cut' material but don't 'remove' material, etched saw 'removes' while 'cutting' so there's no stress on the other leg due to the thickness of the blade (which can break a leg). But given there's plenty of 'block' if there's space to maneuver you can even just break them off with a fine tweezer. I tried cutting and breaking, both seems workable.
I think your issue may be, mostly tool related.
This is what I got to work with:
And the first couple I cut off with minimal loss, see the 'block' below walking figure's feet means there's a fair bit of margin left, I need to trim them!