Learn to prioritize. Make sure you take out lieutenants first, minions second, and bosses last. But disable in the order of boss, lt, minion, it works much better.
Why not bosses then lts then minions? Because lieutenants are often buffing fiends, like Sorcerrers or Shamans or Fungoids and other such nonsense. They make their friends much harder to kill, but thye take a while to apply buffs or their defenses and debuffs rely on toggles. Kill them before this happens, they are useless to their if held and/or dead. Then the minions. They deal constant damage to you, despite their lower power and hp. They become a real nuisance, especially if you are trying to focus on the boss, they whittle you down. Take them out, then the boss has less support. Bosses are often just juggernauts. Hold/disable them first, but kill them last, then they will fall easily.
But make sure to kill anything that still moves before you focus on things that can't move or attack you, they will still do lots of damage.
And in all situations, never be ashamed to run/phase. It can save your life.
