Sockets that are designed to grab the outer part of rounded bolt heads are available, this may be an option. They are fluted/twisted on the inside to grab the bolt while you turn it out. Some general tips be patient and persistent and wear eye protection when drilling, hammering, chiseling spraying penetrating oils etc. and/or grinding.

If you can afford the time, spray an industrial grade penetrating oil on the bolt, strike it with a center punch and walk away. Repeat this multiple times several hours apart.

Can you grind the head off of the bolt, remove the bracket and then grab the remaining stud with pliers?

If you have to drill, try to get reverse drill bits it may just turn it out. If not, be absolutely certain not to drill too deeply, measure and mark the bit with something so you know when to stop. Have someone watch as you drill so that you do not drill on angle.

Had success once with this type of situation by epoxying the hex bit into the bolt head and turned the bolt out the next day after the epoxy cured.

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