I have found the drain plug removal requires a "shock" type torque force rather than constant torque force if you get my meaning. Typically when the bolt breaks free, it cracks or snaps loose very quickly often causing busted knuckles and a fair amount of cussing or even wrench heaving!
The depth of the Allen socket on these button head style bolts is very shallow, so you need to have an Allen wrench in good shape firstly, I have even ground them flat on the end to make sure the wrench gets all of the available socket depth in the bolt head. A few good shock bumps with the heel of your hand on the Allen wrench rather than constant twist force is usually the ticket for breaking them free.
If the Allen socket in the bolt is actually stripped, I have used vice grips if there is access, but for this situation, and more commonly with the frozen or stripped Phillips head shoulder bolts that hold the body panels on, I often revert to a sharp center punch and a hammer used on the outermost diameter of the bolt head. Get a good center punch divot or two started, then a few good strikes with the punch and hammer at a right angle in the ccw direction, and it usually breaks free no problem. Definitely a good idea to replace with a flanged hex head bolt for future consideration as well.
FWIW.........
