1) Fight from a distance when possible, take out other distance attackers first.
2) Sometime you can fight from another level or across a gap where they can’t attack you.
3) Run away when low on health. Return when recharged. You need to do this with bosses.Kill off their helpers first.
4) Poison and entangle work great. Entangle makes the group attack you one on one, where they lose.
Poison up close lets you quickly kill a boss.
5) Upgrade your skills/health with the trainers.
6) Make gems and upgrade your equipment.