Before you try anything soak the bolt for a few days repeatedly in a good release agent like Plusgas. If you can grab the remaining part with a good pair of mole grips go for that, and turn the bolt to tighten slightly the undo but only very little until you see some movement, then apply more release agent. If unable to use mole grips then CAREFULLY drill the bolt and use the biggest ez out you can fit into the bolt, be gentle and do not break the ez out. If that fails progressively drill bigger until you can run a tap into the thread. Not a fun job. Once you hopefully get the old bolt out apply some anti seize to the threads to make it easier to come out and slow down the corrosion of the bolt. I also put a blob of hot melt glue in the head of the bolt to seal that afterwards, easily removed when you need to remove the headlight bolt in the future.