This is an ability you will get out of one of Ms. Ruby's vaults I do not remember off the top of my head but it is either the first level or apart of the first hub area before you unlock the gate.
This has to be 10 DIFFERENT ENEMY TYPES. The same enemies all have the same name to they are not different. you can get 6 in ms ruby's area the spider, the mosquito thing, the weird brown monster, flashlight guard, weird flaming skull dude, and the ghost, and 6 in panda king's hub the one after ms ruby's. the purple monkey ninja, monkey that catapults his kid, the kid that is catapulted by his dad, flashlight guard, pink suit ninja girl, and the monkeys training in the dojo poles.
So in short when you get this ability in ms ruby's area you do not need to back track but it won't hurt you