Phaedo: It sounds like you may be spreading the skill points thin by training a bunch of things at once, or by training some things more than you need to.
My two winning MfIEs (
here and
here) both focused exclusively on Ice Magic in the early dungeon levels. Ozo's Armour, Freeze, and untrained floor trash weapons took them to Temple depth. Then they stopped training Ice (which had reached around 8-10) and picked up about 10 levels of Dodging, while grabbing only a couple of levels of support skills (e.g. Charms/Conj/Summoning to boost starting book spells). By Lair they were ready to focus on weapon skill, while throwing in a bit of Fighting or additional support skills.
That's the typical pattern for any MfIE I've played that had a shot at winning (I've splatted a lot of them in optional endgame areas when I could probably have just won). By the time you reach Lair you should have swapped to the best-enchanted leather armour you've found and be ready to train a weapon skill -- usually meaning polearms since you're basically guaranteed to have a trident by then, but possibly meaning another skill if you happen to have found an endgame-quality weapon of another type with lowish skill requirements. At that point you should be able to rely on melee as your primary offense, with spells for support: Throw Frost/Icicle to soften tougher enemies up, Ozo's to boost defense, and Ice Beasts as backup.
If that's not working for you, maybe the problem is tactics? Merfolk are pretty robust, but they're not as forgiving as Minotaur or DD, and you can't just run away from things like a Spriggan. That means you may actually have to be careful about choosing your fights in the early D levels while you get your magic online; but then those spells can carry you as you branch quickly into melee, at which point the combo always feels rather powerful to me.