I was removing bases from a previously owned rifle that is now mine, and the last screw will not budge. The previous owner or whomever set the bases on used red loktite. Not sure if it is really the loktite holding the screw in or just over torqued.

I dont want to have the flat head screw driver slip off anymore and ruin the slot. Is their anything that i can try to get it loose. Heat, pb blaster, kroil ???? I do plan to find my best fitting flat head screw driver and grind the tip slightly to be sure it is square and not rounded.
Surely lots of folks have been through this. Any help will be greatly appreciated.