Chest Thump was the real killer. It is actually every 20 seconds, but the 38k + stun is brutal.
Haven't killed him yet, but he looks to have at least 6 million hp, probably 7.
His average hit was only 6k on the merc and over half the hits were for 3k or 3.5k, but there were occasional hits for 19k. A 95 healer merc could not keep a 94 tank merc alive for longer than a minute or two, at which time a lucky round would drop the tank.