Go to the top platform of stark tower, there is a coop challenge.
Log in with two controllers and a second account (do not have to be on live, though).
I used Thor on main account and Iron Man on second (doesnt matter, you can use any character really).
Place Iron Man close to the glas fence and then use Thor, throwing his hammer. You might have to charge up the hammer throw.
Once I hit Iron Man with the hammer, he went over the fence and off the tower and thus adding +1 to me. Repeat another 39 times and the achievement pops.
I managed all 40 within the 5 minutes the challenge last.