Place a washer over the hole in which the broken drill bit is lodged. If the broken bit is below the surface, use a ball-peen hammer to gently beat the washer in until the inside of the washer touches the broken bit.
Weld the washer to the broken bit with a wire welder.
Weld a nut to the washer.
Use a socket wrench to turn the welded nut, then pull out the nut, washer and broken drill bit.