Oh Boy.
This means you used either a 12 point , or a terribly worn out Taiwan
socket on them bolts.
Vice Grip has an open jawed tool that will grab onto the bolt it it is
even rounded a bit.
The if that doesn't work cut the head off the bolt off.
Two thing to think about though.
First, there isn't going to be much to grab onto after you have the flex
plate off.
Second,I hate the thought of drilling and screwing up the threads on the
end of a crankshaft.
If you wind up cutting the ends of the bolts off to get the flex plate
off, warm the hell out of the end of the crank prior to trying to twist
the bolts out with the Vice Grips.
This may be your only savior,
Heat.