Gregg is new feature mini-boss that comes in any dungeon( excluding that of legacy dungeons: WO and DT) which gives 3 loot of the highest dungeon and mode you are in.
He will get harder and usually unbeatable above 5-6 players do to leachers, people who wait at the start or just not even enough damage to beat him, so stick to doing it with below 4-5 people who actually fight
