This challenge is the 5th door (second from the right) on the bottom level of the Proving Grounds. The Proving Grounds are on the far right of the Badlands map, most easily accessible by traveling to Villachula, going left to Badlands, and then heading up using the Eagle launch points.
This achievement is easier with a fully upgraded chicken, including one very late-game skill:
There are three rounds of enemies, and the entire challenge must be done using the chicken. The first set of skeleton enemies are straightforward kills using slides left and right, but at the end of the first round three spinning spiky turtles appear. It's difficult to see where these enemies on the ground without getting hit, so I jumped around away from them and then used Pollo Dash to come in and hit them and dash back out. You don't need to kill them quickly, just fast enough to keep your combo counter. This is the toughest part of the achievement.
Once the first round is over, you'll be awarded the bronze medal, and you'll get a big jumping guy - bash him repeatedly with Pollo Shot in the air. If you have your counter up and infinite Pollo Shot, he's trivial. Don't land until he explodes into a rain of little guys, then mop them up with slides back and forth.
As soon as the "Round 3" text disappears, jump up and start Pollo Shot back and forth across the big guy that spawns. Take him out quickly, then Pollo Slide at the bottom to finish up the two little guys.