For the mathematically challenged (me), is there a general, simple, easy-to-remember rule of thumb for when a monster's AC is high enough that you should not use GWM because the -5 to hit becomes too much of a handicap? i.e. Rule of Thumb: Don't use GWM against an opponent with an AC higher than 'X'? Or are there too many variables to make a general rule? If it matters, assume a strength-based generic mid level player with a +4 or +5 to hit.