Quick answer: 6 star Athel and max her out. 

Your War priest is a good tanky healer. I wouldnt recommend taking her past 50.Shaman is nice damage and can ressurect champions. 

More involved answer: After that, decide if you want to spend gems doing Athel's masteries. When I first played, I was getting a lot of free gems from different offers, but I don't know if that is still true. I maxed my gem mine and then bought the 900 gem ancient pack, btu in hindsight, masteries would have been useful as well. Salt on youtube does a lot of beginner videos. Others do as well, but he seems to have the most for the beginner. 

I have never used acolyte, but he can remove debuffs and reduce turn meter. I don't know if he is worth putting books into, but I'll list some free/ low cost champions that you can farm in campaign /buy in market.

Your Spider does decrease def AND weaken in one ability. Never used him, but check ayumilove ( google Raid spider ayumilove) to see the guide on him and others. The site even recommends where they are the most useful with a 5 star rating. 

I would recommend you farm some champions from the campaign. Diabolist (speed buff and turn meter increase), War maiden (defense down), Spirit Host (speed lead and debuff remover) and get into the Arena as soon as possible.

All farmable champions basically mean you can save rare books and just feed them doubles of themselves. Also be on the look out for Armiger and shield Guard. Since they are uncommon, you can buy them in the market as well.

Your Coffin Smasher can be used for clan boss along with Athel. He does HP burn and attack down, Athel does weaken. War maiden can heal. You just need a defense down and a poisoner. Outlaw monk is a great low cost ( market and mystery shard pulls) poisoner until you get kael or Frozen banshee. 

Hope that is not too confusing and is what you were looking for.