I hate to sound elitist or anything, but after watching this this is in the most literal sense a skill issue. The first time the boss raises its arm here, you run to the wrong side even after the arm's up. It's not like the boss raises his arms at random times you have to be on the lookout for, once the orbs come down he starts the combo, and he has a castbar besides. There's also no reason to run through the orbs either (which yeah, that's why you got the poison dot after watching this), just stick with the group.
