This one is pretty self explanatory and will probably come naturally sooner or later. You have got to manage to finish a mission with the maximum score and not having had to revive a team-mate, died, or loaded checkpoint. Using the
button, usually you can see criteria to lose rating on the upper left corner of the screen.
The teamate does not have to be human, you can use the AI as well. I unlocked it quite easily during...
*** Spoiler - click to reveal *** Hulk's mission Condition: Green that unlocks after grabbing JARVIS's neural uplink. I explored the map, killed everything i could (doesn't affect score) and then followed the main objective and rescued a survivor. That was enough for 5 stars in all categories. I wasn't even trying. You pretty much just need to avoid going down/dying