The yellow guards are those pesky ones that move when you do. I grinded this achievement in Level 3-8, where you get the Silverballers, and three yellow guards.
Go left, right, left, left to dispatch the first two with the Silverballers. Next, go up, right, up to take out the final one. Reload and repeat. You may want to get yourself killed to grind for the 200 deaths achievement as well.
Alternatively, you can use Level 3-9 as there are so many yellow guards in that one. It's trickier to do, but once you master the "Kill all Enemies" objective, you can redo until the achievement pops.