Make good use out of your friends and hire high level friends.
When using your friends use the minimum number of your knights you think you can get away with. E.G: I use 1 of my knights and 1 friend instead of 2 knights and 1 friend so the friend is actually used and you save your knights' hp.
Also, focus on enhancing your + armours - once you've got normal armour and hit the + mark try to craft a set and then use the old armour which isn't + for either enhancement or fusion.
And for the arena, double down.
Here's my code: XBB-CNP-XHC - add and use me as you like. ;)