Tool to open the heater

what does the screw look like?

You don't need to remove those triangular screws. Just remove the standard phillips head screws. Slide the plastic "foot" off, & you should gain all the access you need.
If you should determine to go after those triangular screws, anyway, there are two ways. 1. with a dremel cutting disk, cut a slot in the head of the screw, and remove with a regular screwdriver. 2. Sacrifice a cheap screwdriver by putting it to a grinding wheel, and forming it into a triangle.