1. What if like, the Monster is like a mistake that's coming from some weird dimension that is opened up by the portal? And like portal-travel is commonly used in this universe but this is the first time a horrific monster has appeared out of it, like, falling out of it or crawling out of it. 

2. Additional waves would be cool. And maybe we could have a character that is flying high above in a small command ship, observing from below.

3. not sure what this means. is this sci-fi or fantasy? 

4. If it's small, maybe a few vehicles. But if it's a big battle with a big/smaller but powerful monster, definitely bring in some vehicles that can be destroyed.

5. I was thinking that, too, because then we could use this universe as a sort of on-going battle with this monster versus like, the entire planet. Just have different people pick it up or join in every once in a while. With a few twists thrown in, of course. 

Perhaps this first battle would take place in a sort of remote setting?