If it is too tight for a screw driver, I use an impact driver, the type you hammer, not an impact gun. Impact driver works great and you can get one for less than $20.
If you can and don't want to buy the hand held impact tool (which does work well) try catching one of the slot edges with a punch and hammer. Just tap it hard enough to break it free then use a large screw driver for the normal removal after that.
