Just wanted to point out you have to do this in 1 go since the last save. Failing and restarting doesn't get you the cheevo anymore. The last checkpoint is in the last (5th floor) hallway and there's only a few enemies between you and the end of this mission. Hope it helps someone and clears confusion about restarting after failing, then suceeding and not getting the achievement :)
Also you must "flee" after you hold off the Squashtrich.
I didn't have the Rock Band drums or a rapid controller and the lighter/pen gliding tip didn't work for me either so I hold the controller slightly left and just mashed my fingers off, it's a frustrating achievement if you don't have any extra stuff, like the drums, to do this. Just mash X like your life depends on it and you will unlock it after good determination :) It popped for me on my 12th try.
I'll include a little video, not purely about this achievement but it will show you about the Squashtrich and what to do and how it goes if it's your first playtrough so you know what happens.