I tend to drill off the head of the screw.
If you know the screw is an M3 try a 5mm drill bit, if an M4 try a 6mm drill bit etc.. Go nice and slowly and you will get to the point where there isn't enough material left to hold the joint tight and the head will fall of with a gentle wiggle.
The stub that is left is likely to be undone with a pair of pliars or small mole wrench.
If the stud feels tight and is into a metal part use a good soldering iron to put some heat into the joint as this will break down any (sensible) threadlock used and make it much easier to get out.
You can also buy screw extactors which have a reverse spiral thread at one end and conical milling cutter at the other end.
You use the milling cutter end in your power drill to form a particular shaped cone in the rounded head of the screw. Flip the extactor over and run the drill backwards with some solid downwards pressure and it will cut into the new cone form and wind the screw out!!
These aren't cheap but compared to the price of a new chassis they make a god investment.
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