Herald of Titans, to lvl a character to 80, with 9 others and grind the gear for it, not to mention find people with skills good enough.... That's gonna be really tough... I just got Wrecking Ball a few hours earlier at lvl 90 as a Mage, had a good friend following me in AV and we did the reinforcement to 0 tactic, we won and I got the achievement moments before! It was awesome, never thought i would get it.