Warrior has fastest clear speed. Gear for status immunities+HP+passive skills %, weapon should have highest DPS+ignore armor. Take 50% passive skill to gain charge and spam Circular Strikes. Impossible levels drop a lot of purple gear to dress up.
After spending some time with this game, I find it dull when you hit the grind stage. A few urgent fixes - less random crafting results, more control over the type of special mercs you can hire (e.g. have a few mages available at all time), better inventory management system with auto-functions.
The game needs a lot of polishing, but is complex enough to get a 5/5. For later stages, a good strategy is to put a sacrifice unit front center, let the enemy come together on it, and use AoE spells.
Nice game 5/5. Tips to beat it - get staff, max out HP skill, staff tree skills, dodge and +2 movement. Get 1500 and 750hp rings, and regen rings. Staff dmg and regen% bracelets. Keep spamming long range shots and waves, run to regen if you need. Greatsword is medium diffucilty. Dagger is hardest, because of low range and damage you take.