Our world can be destroyed by the Gamma monsters, but first the decided to deal with the S.H.I.E.L.D. headquarters. Defend your base and stop the enemy where it is now. Rotate soldiers to improve your power.
The Gamma Monsters are powerful creatures from the distant planet and some of them are not that easy to kill. Try to defend the S.H.I.E.L.D. headquarters and wait the Avengers to come. Rotate the lines to improve the kill rate and place the captain on the most difficult lane with various monsters to make sure they won't break it through.