Arcanist here, I always wait for the tank to make first move and attract the aggro before attacking, but may I add my two cents for other fellow DPS: don't start running around if you get aggro on yourself, try to stay near the healer and behind the tank so you won't attract more aggro to you, I've had one party in which the other dps started running in circles to try to escape and he ended up attracting more enemies to himself. Healer was foccused on tank so I had to act as secondary healer for that dps :/