Level 3-9, after you've been introduced to the Silverballers, is the best place to grind this one out. There are six patrol guards, and five are easily killable. The sixth takes too long to kill as opposed to restarting the mission.
1) Right, Up, Secret Passage, Kill #1, Up, Kill #2.
2) Right, Left, Up, Kill #3,, Left, Left, Kill #4 and 5.
If you haven't done the Kill All Enemies objective:
3) Right, Right, Down, Down, Secret Passage, Right, Right, Up, Kill #6.
Otherwise, finish the mission for the 10 Turns or Less objective.