Buy, borrow or rent an impact driver to free it up. Or try a Robertson (square head) screwdriver and lean on it and use Visegrips to turn it. Turning it back and forth can often free them up. Also, tapping it with a hammer using the correct screwdriver or with a punch can sometimes also help to break it free. Should only be into OSB or plywood and shouldn't need gorilla force. Shouldn't grind the head or cut it off unless it is severely rusted and is the last resort.
Some toilet flanges are glued into the tank and some screw in.