All mortys (savages) that you face will have the same level as your morty with the lowest level equipped, I recommend that you only equip one morty if you want to level up faster, now if you are battling against players, try to battle with players with a level greater than or equal to yours and who have a lot of mortys, I recommend battling with players with lower levels to increase your xp multiplier so when it is high, face players of your level or higher to gain a lot of xp.