Thanks for the feedback !
The problem i saw while testing bigger wave was that you would have to go to the more efficient way to get rid of the attackers. With only a few, you can do what you want because you have time for it, you can toy with them by wipping the floor with them, you can launch them anywhere, you can enjoy the evisceration animation. It's not the most fun, but hey :)
My reasoning for the limitation is the following: It's not specified what will fail inevitably. Here we have to opposing actions, the tentacle demon trying to open the portal & the attackers trying to close it. For one of them, failure is inevitable. I mean you can't get any dumber than trying to close a doorway between world by hitting it with your sword. So, here, the limitation also comes with "success is inevitable", and the player is not on the losing side x)