Yeahh it could be like that sometimes, me and my friends have that exact same issue too, we're afraid that we couldn't finish it in time. It's just the nature of gamejams haha
I was thinking that some monsters have the ability to summon more monster, or maybe if the player has the ability to dash, you can add monsters that can dash too! Have you played Hyper Light Drifter? If you need more ideas for monster variations, that game will surely give you tons of monster variation ideas!