Leech is perfectly fine, why? Because enemies use it against you in far more powerful variants than you'll ever see. <snip>
But the reality is that the way it worked on those mushrooms is how it should be.....non-freaking-stop. If a bunch of mushrooms can be and are in effect invincible then so should the player's party. <snip>
Ok, Darth, that's the second time I've seen you post about your all-mage party having difficulties with those mushrooms. I have a few points in response to your slightly impractical suggestions.
1. It was one encounter. Keep things in perspective.
2. It is fitting that the game challenges you in different ways and some enemies will have resistance to lots of forms of damage. Some resist poison, some are healed by it, some cannot be stunned or blinded. You have to learn their weaknesses and adapt.
3. You made a party of just mages - therefore leaving yourself very exposed to magic-resistant creatures. Lots of games, not just D:OS, have magic-resistant creatures. Others are very melee resistant (or totally so). In an RPG, you're going to get screwed if you don't keep your toolkit broad-based and flexible.