One of the Arcane Shot options for the Arcane Archer fighter (from Xanathar’s Guide to Everything, p. 28-29) is Piercing Arrow. The description of Piercing Arrow is as follows:
You use transmutation magic to give your arrow an ethereal quality. When you use this option, you don’t make an attack roll for the attack. Instead, the arrow fires forward in a line, which is 1 foot wide and 30 feet long, before disappearing. The arrow passes harmlessly through objects, ignoring cover. Each creature in that line must make a Dexterity saving throw. On a failed save, a creature takes damage as if it were hit by the arrow, plus an extra 1d6 piercing damage. On a successful save, a target takes half as much damage.
My question specifically is how is this handled for enemies that are large than your normal 5’x5′ square? A Tarrasque is said to be 70′ long, and if the arrow goes 30′, then it would pass through 6 5’x5′ squares before it’s done. RAW it seems to say it would just hit it a single time.
So versus a large monster, would this basically be a single shot against a large creature (bow damage + 1d6 piercing) and is more tailored to a line of smaller enemies?
Also, would it be too much of an imbalance to have a creature do multiple saves and damage if it passes through multiple squares’ worth of its space? (I’m thinking of something like the Piercing Ammo or Dragon Piercer from Monster Hunter World.)