Supernatural Abilities (Su)
Supernatural abilities are magical and go away in an antimagic field but are not subject to spell resistance, counterspells, or to being dispelled by dispel magic. Using a supernatural ability is a standard action unless noted otherwise. Supernatural abilities may have a use limit or be usable at will, just like spell-like abilities. However, supernatural abilities do not provoke attacks of opportunity and never require Concentration checks. Unless otherwise noted, a supernatural ability has an effective caster level equal to the creature’s Hit Dice. The saving throw (if any) against a supernatural ability is:
10 + ½ the creature’s HD + the creature’s ability modifier (usually Charisma).
And the beholder...
Eye Rays (Su): Each of a beholder’s ten eye rays resembles a spell cast by a 13th-level caster. Each eye ray has a range of 150 feet and a save DC of 17. The save DCs are Charisma-based.
As (Su) ability and not spell-like they ignore resistance...
So if a Master of Many Forms with at least one Assume Supernatural Ability become a Beholder he can attack without worrying about SR.
I am missing something?