Physical Immunity, with the Catch being against magic (+2 protects only against, +2 very rare to find, +1 or +2 for knowledge of, so thats a +5 or +6 catch, making PhysImm cost 3-4 refresh).  Mimic Ability, spend up to a cost of whatever you can afford.  Maybe "sponsored magic", to represent eating a spell and "recasting" it.  
Also, since this is absolutely a monster, the total adjusted refresh could be very negative.  Even dropping the sponsored magic, lets just assume the catch costs "+6".  That means it has 2 refresh, plus however much you want for Mimic Ability. Lets just say you have it take 4.  Now you have a creature with -6 refresh.  And after "X" refresh, it gains another point of refresh for mimic ability.  Like, since it has 4 refresh worth mimic abilities, it has to eat a caster with more than 4 points of spell casting ability to gain just one more point of refresh.