I'm going to assume that it's not a translation error and that the artist actually forgot the explanation of barrier dude.
It's not *just* absorbing energy from the planted magic crystals. The reason that--in theory, of course--the barrier will hold forever, is that once established, it'll even steal the magical power of the monsters trapped in it. So unless a monster can break it in literally one shot because the initial barrier is too weak, it'll scale in power with all the monsters trying to break out.