This is best done with only one other player. Load the 'Steel Beast' map, as both the human players and Ustanak will spawn right next to each other on the catwalks beneath the bridge.
Choose any character that has a shotgun or magnum (preferably Leon or Agent) and simply kill the Ustanak for the match to end. Allow your partner to do the same during the next match. Repeat this process until you both get the achievement.
Note: While using this method, the player who kills Ustanak will also unlock Invincible and Held Captive in the process.