Why so? My lindurms try to atack bros with lowest pdef, not the tank on a hill, or it is webs that prevents head from moving and providing focus for both head and tail to target tank since tank is only one reachable for head?
It's because the lindwurm still has armor. You only get the acid damage when you hit the lindwurm's hitpoints directly. I don't know the minimum threshold though. So I might as well take a few free pot shots at the lindwurm in that one specific scenario.