Biggest problems for most people in AV:

1.) Healing. Don't know why since I don't heal, but apparently it's rough.
2.) Accidentally pulling extra mobs. The key to fixing this is simply going slow and pulling back. Some rooms it's ok to pull bigger as long as you have CDs available.
3.) Second boss.

Also back in 2.0 when lag was a thing, instant cast Bad Breath on the last boss = fml.