All four in one place
The shell that had appeared in front of Erin to block Laxmus's punch wasn't an ordinary shell, nor was it a beast item. Instead, it was another Familiar and one of the legendary Four Kings known for his defensive shell and massive turtle-like body, Genbu.
Of course though, despite the shell getting more robust over the years as Genbu de-shelled and got new ones, it was nothing in front of a being like Laxmus's fists because the latter had enough strength to destroy mountains and quite possibly the Earth itself if hit and directed at the right spot.
However, Erin had already predicted this but still used the shell for a reason that it was a backup plan, and it did work because the shell turned out to be strong enough to give her a few moments to get a hold of herself.