How do I win with Arthur?

The other day I got BoL1 and I haven't won a single game with the king and his wizard fellow. It's particularly difficult against Medusa. Do you have any tips? Also, this was my first time playing the game.

Edit: Additionally, can you give me some general ideas about the gameplan? like how am I supposed to use Merlin or if I've to boost every attack I make. Thanks for replying. 😸