Some more explanation and another trap later, Doofenshmirtz obtained the amulet and with it, he activated the colossus just before Perry from 1914 arrived. The colossus still managed to wake up and started to follow the team and the recently saved Isabella and New Hampshire Garcia-Shapiro.
Once they were at the top, the colossus was shown to still be going after them despite being on fire from one of the traps from earlier. Then Perry's hat arrived with the amulet, and the team used it to make the colossus explode, killing him in the process.
The Corn Colossus is a giant yellow maya statue-like creature. He might also have been made of corn itself, since he exploded into popcorn with the amulet.