This one is either buggy, confusing, or just unintentionally missed, or maybe all three of these things! I only unlocked this on my way to finding and completing level 6.9 after having a quick brainstorm as to how to actually use all of Putty's attack power-ups.
I thought to myself: "What if the 80-star power-up, which is the one that fires the grey walking putties at enemies, is the last attack that Putty has. Maybe I need to downgrade this power-up and see if there are any lower ones I'd not used!"
Standing to the right most side of level 6.7, I spammed all of my grey putties until the meter to the side drained. Then my electric finger attack, then my blue arrow attacks, then the lowest power was the putty boxing glove. At this point, the achievement didn't unlock, so I just left it at that, quit the game, went back into marathon mode and reloaded the level I was on.
My putty was back at the grey putty attack and as soon as I fired one grey putty, low and behold, the achievement unlocked!
Check out the video to see exactly how it went down!