Or file flats onto the stud then use a spanner.
Try to get it out as it is first: drilling & extracting is last resort if there's an alternative

There's a product called freeze-spray I sometimes use at work for stiff bolts. Spray this on and it contracts the item enough to break any adhesion.

An example: Maplins, £6
http://www.maplin.co.uk/Module.aspx?...reezer%20Spray