Earth moves do have an problem where if you go beyond a certain size, you're just making an elevator. He doesn't seem to throw the rocks.

That said, he was using an earth dome/shield against ice projectiles before. I actually thought that was a pretty smart move given how barriers are supposed to be relatively ineffective against physical attacks. In that case he should have used that against the water spirit bomb instead of the usual barrier - time permitting.

Another inconsistency I've seen so far is that in some fights, a barrier blocking an attack can deplete your mana completely (like if you blocked an attack that was too strong in Dark Souls, it'd break your stamina). However, when one of the rock pillars struck water girl from the ground, it broke her shield then damaged her - but that strike didn't automatically drain all of her MP compared to say Denken's fight and Richter's final shield. That would suggest that you can break through a barrier without exhausting someone's MP. One explanation for this is that you are both limited by both mana capacity and mana output - and water girl maxed her mana output into the shield, but not her capacity. Frieren states in episode 2 that long range spells are affected by mana, strength of firing and control. I'm guessing shields may also apply similarly - and Denken/Richter's output is high/instant enough that their shields are limited by their mana capacity instead of rate-of-discharge.

To be entirely honest though, analytics aside I don't think these battles were designed to follow strict mechanics.