Buy a postern gate then put some weak soldiers outside. Also get a hero with arrows e.g. Legolas. Wait till some enemys come then run the weak soldiers back threw the postern gate. Legolas can then kill the baddies as they run away. p.s. the longer you leave the weak soldiers outside the more time youll have to kill the baddies. (or you can leave them there to die.)

Another tactic is to use aragon instead of the weak ones. He will get level ups too. This is a good method to get hero levels up quick.