1. By this universe do you mean the monsters? That would make the most sense. Portal travel does exist in my setting but is rare or special circumstances. 

2. So scout planes that get "king konged" out of the air. Could work. They have plenty of anti-gravity vehicles I could come up with. 

3. Sci-fi with magic. Their magic follows rules rather than, "because it's magic I shall spring forth this magic fireball in your face!"

4. Smaller like 30 feet is better so foot soldiers have something to do and some who wield swords can do cool flips and such onto it's body and do some damage.

5. Yeah. And the monster destroys the planet in the end. NOOOOOOOOOOOO! Perhaps the first battle is on a rocky beach. The planet would have diverse climate so it's not all the same type of battlefield and getting boring.

6. A villain fighting through all my heroes would be a good plot as well, like the monster going through planet locations idea.