Use your 12pt triple square bit that you have for the axles and hammer it in. It will come out easily.
You beat me to it.
I keep a worn out one just for this.
I think I got it to work on stripped brake caliper allen heads too.
just drill the head off the bolt then twist out the rest with your fingers..
Not these little fingers lol......
These bolts are in there pretty good im gonna go out and buy a new set of vice grips and may even do the left handed drill bit trick too.... Wish i had an air chisel, this car is def forcing me to buy tools as i need them lol.... 
if you drill the head of the bolt off, there is nothing more than a "stud" sticking out of the crank sprocket.. then you can slip the pulleys off and un thread whats left of the bolt.. when you remove the bolt head, there is no more load on the bolt, it simply un screws with your fingers or a pair of vice grips..