OMG! I've done it before! and this is probably the best thing you can try as a naga! I was seriously considering it again when the platinums rolled out.
Even though sacrificing melee is almost unthinkable to most powerhouses out there who kills faster than they move, naga mage is a good combination and makes you all but invulnerable
The 15 fire is a great idea, and I say put in some FW since it benefits both melee and magic... but strength is what smacks reapers... so, find a good balance.
At the time I tried it, I was a water mage since I was a dragon till level 27+ ... water is a poor element choice and nobody knew anything about FW at the time
As for the mage / unguilded thing, just think of mage as a (really) expensive alternative to unguilded.. you can always overtrain 5 unarmed or so before unguilding and joining the mages
um... the five necks for amulets and crowns offers a lot of opportunities, but don't bother with switching amulets for casting resists...they take waaaaay too long and you'll have fire recoil immune anyway which is what matters most. earth magic or coran amulets for casting strength is definitely worth it though.
Oh, and prismatic shield is your friend.
- consider trying the berserk spell (use extreme caution, but the positive results are unimaginable)
Oh! and keep some agility! ghasts and other meleers will make quick work of you if you don't ... maybe a single plat girdle will do, or maybe consider getting some plat helms for confusion immune.
Keep us updated, I really want to know how it turns out
