Shell Kickers are what Michelangelo calls the 4 different combo enders.
When you find the T-Phone upgrade in the TCRI building you get the option to use you experience to upgrade each of the 4 combo enders for each of the 4 turtles.
To get this achievement you have to simply not upgrade anyone.
I ended up getting this on my 1 hour speedrun. I didn't even pick up the t-phone upgrade during the run. The upgrades are not needed to beat the game.