got stuck on level six for ages because i didn't realise that i could recruit priests at that stage because it was greyed out, i felt so gimpy because i couldn't beat the stage lol.
Tip to beat the boss for anyone that's struggling. Max out all of the character upgrades and max out all of the runes before the level starts. This helped me a lot. Good luck.
 "Royal Warfare"){kind=icon}
 "Mighty Knight"){kind=icon}