First thing to do (after applying an aggressive penetrant) is to impact the periphery of the cap repeatedly with a wood dowel. Work it repeatedly until the threads are cracked loose from each other. There will still be static tension on the threads that will require some torque to overcome.

Using a 7/32" or equivalent 5.5mm hex key, cut off the short leg of the key (and the bend with it).
Taper the tip of the key to the tune of a half-millimeter, using a flat file to taper each of the six flats.
Now you have a tool that can be effectively driven into the rounded hole, recreating a good part of the hex shape that you once had there. Grip the key with a Vise-Grip while striking with a hammer while using eye protection, then turn the cap out of the crankarm.
The cap can be re-used, but you will want to force the tool a ways through the hole (with the cap perhaps resting on a block of wood) so that a normal 7/32" key can later be used to install it and remove it in normal fashion.
Grease the cap threads well before reinstalling!