Petrified enemies will block enemy movement, and you can still get mana from their destruction, by you or enemies. Petrifying a large enemy is not only safer than fighting them - it also gets you more mana back from their destruction.
Arrows are very dangerous - use stoneskin whenever there's an archer around. (I'd intended to implement a more interesting mechanic here, but didn't get to it in time.)
Stoneskin won't stop the rocks from sling orcs, so it's usually worth the cost to just petrify them on sight (or torch them, if enough other enemies are suitably positioned.)
Melee combat is the only way to get "free" mana early on, and smaller enemies will die in one hit - don't be afraid to use up your transfusion imps.
Later, there's also free mana from arrows that miss you and hit enemies.
Using the exquisite fan will get you a lot of mana if you can make it hit enough enemies across a large enough arc. Torch is very effective cast along a bridge full of enemies.
I hope that helps. Thanks for giving my 7drl a try! :)
