This trophy is titled "The Best Around" and it requires you to earn a Mission Score (5 Stars) of 100% while on Challenge IV difficulty.
My suggestion for attempting this trophy is a mission called Data Squib Repository in the Snowy Tundra. There's only a single section to complete (capture and hold the three terminals), reducing the mission length as well as the time where you can go down. My suggested Avenger to use is Iron Man; he can fly around to quickly recapture points and stay off the ground, away from the mass of spawning enemies. Thor should technically also work, but I haven't used him too much. Kamala might be a decent alternative because High Five can clear enemies from in front of her and Healing Spirit can be used in a pinch to reheal, but she can't fly. That would have her being quite the target for many of the spawns.
From the mission table, select Snowy Tundra, scroll to Data Squib Repository and press the Square Button to bring up the difficulty level. Change it to Challenge IV, back out, and begin the mission.
Note: I am in the end game (Avengers Initiative) and don't know when this mission appears. Sorry for the lack of information on if it's there from the start, or if there's a chain that needs to be completed.
You'll spawn at the start of a long hallway which will have some enemies in it. Stay in the room and use long range attacks to defeat them even as they advance on you. Enter the hallway to trigger an enemy spawn, then fall back into the starting room while using long range. This is to minimize your chance of going down, instantly ruining the 100%. Defeat the wave of enemies and proceed down the hallway (don't forget to collect the resources and chest in the hallway).
For the main room, quickly capture all three points, defeating an enemy or two as you do so. Once all three are done, you basically just want to be on the defensive to maintain your control to end the mission as quickly as possible. Point A is a decent place to hold up every so often. It's an elevated platform, meaning the enemies below can't see you to shoot. They also need to stop attacking your allies to jump up there, giving you a few seconds to damage them as they complete their climbing animation. It's important though to know that if two or more enemies begin advancing on you, think about flying away as damage comes fast and hard. As the enemies begin to recapture, if you're Iron Man, fly towards the point while shooting your repulsors. That will break the connection and you can continue flying elsewhere, or go and defeat that enemy.
That's basically it. It's a really straight forward mission that's just really short. It's just a lot of enemies spawn in and a flying Avenger is the most beneficial in my opinion to keeping you away from most of them.
Good luck.