- Are you using a metric bit? A 1/8 should fit a bit tighter than a 3mm. Use a fresh, quality hardened bit, not some one-hung-lo brand.
- Sometimes a torx offers a little more bite in a boogered up hex.
- Long nose vise grips from the side.
- Use a file to cut a slot in the top of the bolt head. A 1/4" Hollow ground Flat head bit with a wrench grabbing from the side. Lookup Chapman Wrench to get a better idea of the setup.
- I would avoid drilling until you are out of options. If you do drill, use a lefthand drill bit.