So what if you had to deal with so many enemies before you can get to the mission objective? Well, what about self-controlling weapons? (Aka a rotary cannon, drones etc). It would be SO much easier. Or maybe even just put in AI humans or something. I just think it's cool.
Here's the weapons I'm talking about:
-Rotary cannons
-drones
-missile launchers(except, you know, they're self controlling)
-AA guns(for ground vehicles)

And you could customise:
-who they attack
-how they attack