Hopefully you have all or most of the stud multipliers before going after this achievement. Here are the levels that they are located in:
Studs x2 - 1,000,000 studs (Struck Off the List)
Studs x4 - 2,000,000 studs (A Loki Entrance)
Studs x6 - 4,000,000 studs (Rail Hydra)
Studs x8 - 6,000,000 studs (Shakespeare in the Park)
Studs x10 - 8,000,000 studs (Helicarrier Havoc)
Also, here is a video to locate them:
Credit to AFGuidesHD.
After that you have to grind collecting millions of studs to buy them and then turn them on before entering the level and you should hit true avenger status quickly.
Here is a walkthrough video of the level:
Credit to ChaotiXGaming 004.