Without seeing it it is hard to tell.But the first thing I try with any stripped screw is putting some valve grinding compound on the head
.It's abrasive and helps you get a little extra grip on the fastener and works more times than not if used when it first shows signs of stripping.It is one of those cheap
oldschool tricks that just works and is cheap
to have on hand.
After that it is usually vicegrips but you have to be careful around gelcoat
Another trick i use often is i have a set of lefthand twist drills.I use them to drill into the stripped fastener and at times I get lucky and the screw/bolt will walk itself out.But for the stuff you described I doubt I'd use these but still worth having on board ;-)
Chances are it's going to be vicegrips on what you mentioned.But if you can get it to come out a hair it'll let you grab onto it without doing more damage.
Oops,I thought he was trying to remove stripped screws ;-)