[OH&S]

In general there are a few methods one could use to defeat Accelerator:
*Use his reflection against him; Requires intricate knowledge of his Personal Reality and/or superhuman reflexes. Personally, while I think this is theoretically possible, actually implementing it is completely ridiculous.

*attack him directly with vectors he cannot grasp (not inputted in his Personal Reality); Very rare. Not an option for Mikoto.

*attack him indirectly through other means which bypass his reflection eg Food Poisoning etc; The Sisters eventually manage to find a rather effective method of doing so...

*attack with phenomenon which do not function through vectors ie vectorless attacks; I will leave this to your imagination... To my knowledge, not an option for Mikoto

*attack the very foundation of his ability (AIM, Personal Reality); includes but is not limited to Kamijou Touma's Imagine Breaker. Capacity Down would also be an effective strategy against him. Not an option for Mikoto