This can be done during any part of the campaign. It can easily be completed in co-op or versus with another player. It can be done solo, but it is a lot harder to steal a kill from one of the AI. To do this, take cover across from an enemy on a shallow object. Press Forward, then X to pull them over the cover. Once they are pulled over, stay close to them and press Y to execute them. While playing in co-op, have one player pull an enemy over cover, then have the other player execute them to get the "Whoomp, There It Is!" achievement. Have the player that did not get the achievement do the execution next so both players get the achievement. An easy place to do this is during the "Yank & Shank" tutorial in Act I - Chapter 1.