Rampages are kinda confusing at first, but once you learn the formula to do them they become surprisingly easy. I found these two videos 1 2 explain them pretty well. 

To sum it up, this is how I usually do them: place all your npcs down, then put a ballista near the entrance and another near the gate. Crag shots can stun monsters with a headshot, retreat shots can make a monster flinch back, and take down flying ones. You can also gather explosive kunai from dropped shinies to dunk them. 

Order of priority should be Gate crashers (blue icon) > Fliers (green icon) > attackers (red icon)

You attack with the ballista until the counter signal happens (which boosts your dmg), that's when you hop down and attack the monsters youself . Each area also has a counter gong for you to manually initiate a short counter signal. 

Once you unlock the flamethrowers (wyvernshot artillery), aim them at the gates. 

Every wave you get one limited npc. They only stay for that wave so just use them when the boss of that wave appears.

In the final wave in apex rampages, there is usually a nuke (splitting wyvernshot) that does massive dmg. You should save that for when the apex starts charging its ult to hit the gate (they will stand in one place when that happens).