If GANON can create a Sun of Nothing and not get killed by it, he presumably has some rather absurd MDEF, either that or it has a bull**** "does not hurt caster" clause in there somewhere. Given that his abilities are open to interpretation and that clauses like that are universally bull****, I choose to take it as him having quite good MDEF.
Dunno about psychic attacks though, I generally wouldn't rule that under MDEF but rather... willpower or some such.
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"The courtroom is the garden of holy judgment. Those with lechery in their hearts should leave this sanctuary at once!" ~Franziska von Karma