Simple get a high magicka and some good enchantments. They are meant for master mages so they should be difficult to cast.
But they should at least be useful. Pretty much none of the destruction master spells are good, save maybe for Lightning Storm, but even then you're better off stagger locking your foes. The fire and ice spells are pointless, though. Don't bother casting them.
Thralls are okay, if that's your thing. Mostly a playstyle thing, so I really can't ocmplain or agree with them.
Never been a fan of the mage armor spell types, so needless to say dragonskin's kinda underwhelming for me. Maybe if it added some resistances too, but just damage reduction alone? Physical only? I guess maybe for archers, but the only time they're trouble is when they surprise me anyways. Mages always tend to be the real threat to me, regardless of my build.
From what I've heard, the other spells aren't too good either. Maybe Mass Paralyze, but the rest? Eeeeh...
Expert and Adept spells just seem way more useful. Plus, with only those, I can focus less on single school reduction and spread out my fortify enchants, since they don't cost nearly as much to cast.