The game with the current mechanics can’t really support mages getting as strong as skellies without making them extremely OP in Normal/NM. This is because of the way additional damage works. For physical damage there are a lot of items, runewords, skills that add a % of extra damage while only one thing, amp damage, that lowers the monsters physical resistance. We can stack a ton of % to extra damage because we have the aura’s, etc to do so.
Milox’s post below outlines it nicely.
On the elemental damage side there are only items/skills that lower the monsters resistances but nothing that will add a % damage to elemental summons. This means we can only lower the resists of a monster to -100 and we hit a hard cap on the ‘additional’ damage that elemental summons do. You could potentially make them stronger from the start but that will lead to very much OP mages once you drop 20 pts into them by the end of normal.
I’ve been thinking about adding in a ‘mastery’ to the mages after lvl 20. So at lvl 21 mages you get something similar to fire mastery adding a % extra damage for more points over 20. This makes them stronger only with additional points to a maxed skill.