Ninjas are easy to fight when you're level 2 so no problem ^-^
About the Exp gain I think it need to be a little increased ,same for the gold gain ,not a lot ,just a little ,because for example ,at level 1 fighting against Cretins only give 11 exp and like 8 gold ,and you have to reach 2001 for move to level 2 ,or 1000 gold for some classes + the stuff + potions and all ^-^"
Thanks to Warcraft and other mmorpg ,I have the habit to farm so it's not so bad ,but I think at a highest level it can be really long to level up or gain enough gold for buy some strong things ^-^
But luckily ,the ninja give more exp and gold ,and they are easy to farm at level 2 🙂