I've been doing math...
In a story I like, the humans need to kill [5 x their level] enemies at their same level to level up.
Theoretically, this means that a human would need to kill 5 humans to reach level 2.
A level two human would need to kill 10 level 2 humans- each of which had to kill 5 humans to get where they are.
This means that 60 humans had to die for one to reach level 3...