Dude, the M18 would have been pretty easy to kill in real life. Even a .50 cal could have penetrated it from the front.
M18 was supposed to rely on speed to survive, not armor. Learn to use the speed, because thats all you'll get.
As for the flacker, if you're talking about shooting it in the turret multiple times, or shooting it in the tracks, I think thats intentional. IIRC, HTC changed it so you couldn't kill flackers by shooting it in the turret (about 1/2 its target profile) multiple times, even when it was just sheet metal, and (I don't think) ammunition wasn't stored 'in' the turret.