This achievement is unlocked by completing the first story arc named "HAMMER". This arc contains 5 different chapters with a total of 46 missions to complete. Took me a total of 2-3hrs to complete.
Enimies are pretty easy, so I would suggest you get a feel for each characters ability and rank up those you think are best. You can even replay missions to gain XP and level up a bit to make future missions easier.
I personally used the load out of Iron Man, Wolverine/Juggernaut (after unlocking him after chapter 2), and Thor Goddess of Thunder (which later I switched to a better load out, but for this chapter, she is best).