Let's Temporarily

The two elemental pairs would be mainly for damage dealing. If you really wanted defense, you'd get the holy/resist set.

On top of that I'd rebalance the damage formula so that 1 magic attack is worth more than 1 INT.
(1 INT = 0.9 magic, 1 MATK = 1.5?)

I say .7 or .8ish. It doesn't really sound like .1 reduction per point would make much difference, but a bigger reduction would make more space between m.att gain and int gain.

Anyway, my own suggestion, instead of overhauling the entire system (which would pineapple over mages around all versions) just make a weapon set (or just a weapon) that makes the lost int worth it in equal m.att gain. Level 130 staff with 120 req and a base m.att of 260 i guess? Equality, as far as i see. Might need to make the req higher, since it's really pretty easy to go low luck and the extra int would give more benefits still.