The fastest way I found to do this is on 3-5. Roughly 25-30 seconds for 4 kills. I've broken this up into segments, but it's a simple strategy, plus it gets the Kill All Enemies objective.
1) Right, Up, Left, throw can Left.
2) Right, Up, Up, Left, throw can Left.
3) Right, Down, Left, secret passage.
4) Left, Left, kill #1, throw can Right.
5) Right, Up, kill #2, Down, kill #3.
6) Down, Down, Left, kill #4, Down.