Level up world

This is level up Civilization, everyone is a level 0 right now, theres many wars, and kingdoms, stronger ones bully weaklings and to level up, you need to kill others, and training, but training does a lot less EXP, and the more someone levels up, the slower they have to level up and the monsters could go from F to E to D to C to B to A to S to SS to SSS and then to Z but also F to Z Tier weapons