If only there was some place you could shoot it that isn't covered with the shell. Maybe some vital place that, when shot, would kill it instantly, that's not covered by the shell. HMMMMMMMMMMMMMMMMMMMMMMM 🤔
Its in the middle ages guys, they're ontop of a wall like 1.5x higher than where the monsters head is. The soldiers barely have basic training and the shell could be much larger than the artwork shows and cover its head, I don't think getting poor shots onto its legs will suddenly kill the monster or be good use of their limited ammo.
all you guys saying shoot the heads clearly havent thought about the possibility that the heads are just dummies amd the real head was the shell all along /s