Some tips and tricks:
1. Learn to Kite Enemies especially when you have Damage over Time spells like Swarm.
2. Arc Spells (Flame Arc and Icy Wind) can go through walls. Use this to your advantage.
3. If you have Charm use it on an enemy who can heal you. It will save you many times. If there are no enemies who can heal, either target an enemy who is by far stronger, or conserve mana.
4. Try to pick off enemies, especially if they are in a group. Get close and try to draw just one or two and fight them in a room you have already cleared.
5. Target the Healers. They are a HUGE pain and can really ruin your day.
6. If you can Entangle someone in a multi-enemy fight, you can run to another room and they won't chase you.
7. The enemies who use their version of charge can charge through walls if they have had recent vision of you AND can charge out of entangle.
TIP: If you completely clear a floor before you move on to the next one, you can retreat to that floor and completely heal. This essentially acts like a 3rd Gold Relic.