Cannons at Castrum

ok i've been having this problem in this dungeon since the very beginning and i've been meaning to ask this question for a long time. how can i get these cannons to, not work, but cause damage to what i'm aiming at??

i get the orange target/reticle, try my best to have the enemy i'm aiming at in the center and, no kidding, i'm always missing basically every single shot. what am i doing wrong? when i hit i can see the orange damage numbers, but when i miss, i dont see any damage and all it does it aggro them. it's a bit embarassing at this point seeing as how i've been playing since early access in august and i still dont know how to operate these things.