Couple options and I'll rely on others to give you other advice on how to do this..
1.. Vice grip. Front or back, sometimes back is easiet.
2. heat, more heat, soak, Parafin wax works well too.. heat, touch the wax to it, wax will soak in and help.. PB Blaster works as well but bit messier.
3. Left handed drill bits.. Tricky but works wonders.. Drill down center with left handed drill bits, it might just grab it and pull it out or at least leaves you a hole to break an eazyout in there..
4. Eazy out.. Dunno why they call em easy, they suck for me.. And if they break the steel in the easy out is harder and stronger than any drill bit you have..
5.. Welder.. If you can weld a nut to what's remaining that works very well.. Then the heat from the weld penetrates the bolt..
6. TORCH.. Smoke wrench.. Gas ax.. get it hot, douse it, will break the bond
Combo of many of the above processes works.
And if all else fails.. A nutzert tool is your friend.. as is antiseize..
