Dude !!! Don't even act like that boss has never chucked you into the abyss before. I caused a party wipe the first time I did that FP because I was healing and I didn't know about that knock back, and the tank didn't get him all the way to the wall. I caused another party wipe on that boss in HM because I backed off the edge trying to make sure I was out of range.
Just admit you glossed right over all the other stupid stuff in the post, and went straight for "NEWB FLESH !!!!" when you saw how wrong he was about his ammo using heals. I'm sure they were all very flustered by what had just happened.
They had a Sentinel tank? Okay- so the OP doesn't know his ACs at all, got confused about the name of the tank's AC, or none of the players in that group knew jack about what a tank is for. And the tank appearently didn't even understand what killed him.
This event probably went down in Story Mode as well. It wouldn't surprise me at all if he got to cast was hammer shot before it happened. If he cast anything else, his ammo probably regenerated while the tank was stuck in the loading screen.
The OP's story is just a funny fail on so many levels. If I had been healing that group, I would have been paralyzed with laughter when they all went off the side. I have seen it happen before, It's hilarious! Cracks me up everytime I see somebody get pwned by that lizard's knock back. I have watched many a noob melee dps take flight because they thought to themselves, "Dis healorz OP! I will stand in dat red AoE thingy and burst in Tregg's face, NAO!!", after the tank explained to them what would happen if they didn't let them position the boss correctly first.
To the OP: don't sweat it... everybody has to be a noob sometime, and some people are jackhats. You will have a lot better experiences than that one, just keep playing. Now you know how the mechnic works on that boss.