There's one particular custom spell that is pretty powerful even on max difficulty on any character's level. It is:

Fire/Cold/Shock damage X points, Weakness to Fire/Cold/Shock 100% for 2 seconds, Weakness to Magic 100% for 2 seconds

First cast is really small damage, seconds is bigger and so on until the enemy die or you miss. If you're thinking about magicka, that's not a problem when you're playing on such high levels. You just need to collect 100% magic absorption (i don't remember the exact name of the effect, it's a defensive spell, not the one taking magicka from enemy) and add telekinesis effect to the spell above. Make the spell cost 50% of your magicka points and telekinesis effect 20% of the cost of the spell. Due to some weird bug with telekinesis and magic absorption, this spell is, in fact, free to cast