Choose Select Level
Go to Chapter 6-1
Run forward to the right, drop down, and then run to the left. Draw a horizontal branch, and hit it with fire. Then erase the end of it, and drag it to the door. Blow the door up with the fire from the branch. Go through the door and jump at the last two stairs (you will be sliding, and if you jump too early or too late, you will end up in the lava), and then push Y on the lever.
On this next bit, you will find one bomber in the top right corner. Kill him with the fire, and move towards the middle of the room, but don't step on the button. And then one henchman will spawn at the bottom right corner and run at you. Kill him, and then another henchman will run through the door you came from on the left side (which is why you moved towards the middle to give you enough space to avoid dying). Kill him and another henchman will spawn from the right again and runs at you. Kill him and another 2 henchmen will spawn on the left side and run through the door at you. They are fairly close to each other, so if you do not have it already, this is also a good place to get 2 kills in one for the achievement, "Deadshot."
There are various ways to die to restart. You can use the fire to blow up the bomb like I showed in the video, or you can keep him alive to kill you after killing the one henchman. Or you can kill the bomber, and the henchmen, until you reach the last two from the left, and kill one, and let the other kill you. Or if you want, you can kill them all, and then go through the door that you came from, and jump in the lava near the stairs where you made the jump.
Best to attempt after the game is completed and all achievements won, as I only had 18 of Mustacho's Army men left when I started this.
Here is a video I made to show you how I did the last 18 of my Mustacho's Army men after I had beaten the entire game and done the other achievements with gamerscore.