A large flat head screwdriver works, but slow going. Once out you can replace with regular screws/bolts with more orthodox heads like torx or pozi.

 

 

I used a grinder and a dremel to make space for a flat screwdriver.

 

 

oh, I forgot, I used this method to remove first the screws from the bracket on the engine's ECU. First I took of the bolts of the plastic cover same way Sean told, then I used the grinder to make them more easy to be handled.