Zargala is quite good for arena, less so for other places. Same for Candraphon. You have a very strong potential arena team - Seeker, Zargala, Kael, Apoth. Even better once you get Khatun, so you can drop Apoth from that team. That'll easily get you to gold with proper gearing.

For CB, you have some of the key pieces you'll need - Frozen Banshee is an absolute must, Kaiden is not bad either. Goldgleam is good as well, for leech, def down, and weaken. Thenasil could work, though he's pretty weak overall.

In terms of who to max next - I would say you should focus on CB, so I would max Frozen Banshee next. Maybe by the time she's maxed, you'll pick up some other important pieces - probably Khatun would be your next one to max, once you get her.

I agree with 6starring Apo then Zargala, though maybe Zargala should be 1st.  Looks like you are progressing well, so with 12.3 brutal you should be doing both fairly soon.  Kaden, Rearguard, and FB would be next priorities imo.  I think Dragon progress more important for you than CB, though they somewhat go hand in hand.  But you have great champs, you need gear more than shards for sure.  Obviously u need books.  I think seeker and hk (should have soon), will serve u at 40 in short term.

Arena team today: Spirithost Apo Zarg Kael

Arena team tomorrow: HK (40) Seeker (40) Zarg Kael

I just wrote that, then realized you had candraphon... a good problem to have.  

Zargala first, yeah - maybe Apo to 50 first, but then Apo can stay at 50 for a while, while you level up others who benefit more from the jump to level 60. 

And yes, with Candraphon as future nuker and FB as future CB poisoner, Kael's days as part of your core team are numbered, it seems... 

Kael may be best served in dungeons, even after you replace with FB in clan boss and Candra as nuker, in lifesteal gear and an accuracy banner he may be better to get you to dragon20 since FB is a liability until u get to the dragon.   I ALWAYS say 60s before 50s, but you have so many that are automatic 6stars it is not a mistake to 5* them... wow never thought I'd say that.  5* Apo is definitely the next move.