The nerfing of the Monomimus was because they decided it was too easy to make for the power it brought to the table. Basically, almost every battle incubator showers us with Gallimus. The only component needed to make him was the epic found in the wild.
Believe me I feel your pain. When they did the nerfing, I was not yet in the arena events that showered us with gallimus DNA. I spent money on incubators to get Gallimus, and did every green tower event that gallimus was a common of choice. That was a painful lesson. But I understood why.