Teenage Mutant Ninja Turtles Danger of the Ooze

Obviously, it is the most helpful to work on this throughout your playthrough and kill as many as you can while working through the game. I still hadn't popped it after I finished so I did some grinding.

At the bottom of the map is a long hallway that connects the east and west sewers together, it will become available after defeating the Newtralizer and you are on your way to fight the Shredder. In the left room of this long tunnel is a walkway where you will encounter two sets of Kraang droids: one blue droid and one with a shield, if you drop below the walkway there will be 4 mousers to kill. Afterwards exit through the left door and there will be 2 mousers in a shallow pool and then 1 more mouser slightly more left. Just keep bouncing between these rooms and if you haven't killed 300 Kraang droids this is a good place to pick that up as well.

I am unsure if your kill total carries across separate playthroughs (ie your collectible playthrough and your speed run playthrough), so I finished it on my collectible/100% run just to be sure.