Try raising Luck. The Luck stat, I believe, does the following two things:
1. Increases your defense against magic.
2. Gives you a chance (5% per point, max of 95%) of not dying when you would otherwise be killed by damage. (This actually does feel game-breaking at high values, but getting your Luck that high takes a *lot* of skill points.)

(Note that, in the remake, point 2 no longer applies.)

I'm not sure if mid-game is different than the beginning, I had couple of encounters (lvl 8-10 party) with bats in Bat Cave. At first the killed all 4 chars with fire spells. Then I reloaded my save and got for everyone shield spell from priest and haste spell from mage. This way with shield fire damage wasn't enough to kill 1 char with 1 shot and my priest mass healed everyone while others took bats one by one.
try to buff your party with shield and haste before battle, it really helps.

edit:
just wanted to clarify why shield effect is useful. if your priest can cast it at lvl 3 it will give 2 effects - shield and protect from magic. That's why I mentioned it. Also worth trying to spawn many shadows/minions so they will take damage instead of your party.