Your arena team is very good and Apothecary would not improve it. He gives a spd buff, what High Khatun allready does, and 15% tunrmeter filling, while Seeker fills the turnmeter by 30%. Apo and Spirithost instead of Seeker and Khatun would give you more survivability in the fights you can not win with your first hit, but you sacrifice to much speed. 

You should bring Seeker and Warmaiden to lvl 60 to improve your team. Both of them can also fight in dungeons. 

On the long run you should replace Warmaiden with Spider, but he needs to have a booked A3 definetly. Otherwise he has only a 60% chance placing the def debuff. 

Thanks Skadi!

I wasn't sure Warmaiden was worth bringing to 60. FYI, I also have Armina, who's awesome, but I didn't have enough books for her A2 (decrease DEF) so I decided to go along with Warmaiden.

Re: Arena, I feel like my Kael doesn't nuke strong enough. He still has the same gear as I posted above, a while ago, just because I have never found better one. It's a full Lifesteal, 4* at level 16, so I'm wasting 2 spots and am not being super efficient in terms of maximisation, but the substats are so perfect that everytime I wanna change a piece, it doesn't do it. What I'd need are a couple of good Crit Damage pieces, but they're scarce (and I'm only at Fire Knight 11).

6 staring Warmaiden was a good move, but I still struggle in Arena... Until I substituted Seeker for Rector Drath! She really is a game changer. She can decrease ATK on the nuker in the first round, and revive or heal if the fight lasts. My problem is my Kael isn't strong enough to nuke other lvl 60 in one round, so I need durability, and my Drath (who's tanky, fast and resistant) offers me exactly this. I might 6 star her even before Apothecary, as she carries me in dungeons too. However, I still can't beat most teams with 6 stars Legos, which is the majority, even in Bronze I (as everybody, I think the Arena is broken). I don't see how I'll be able to reach Silver I anytime soon...

My current team is : Kael 60 (full masteries), Warmaiden 60 (2nd tier masteries), Drath 50 (2nd tier masteries), Apothecary 50 (5th tier masteries), and Coldheart 50 (2nd tier masteries) + Seeker 50 (for Fire Knight) + Gnarlhorn 40 (for Spider). All fully ascended and booked of course.

I unlocked every Dungeon 13, Spider's Den 10, but not with 100% efficiency, and I feel like a bigger Drath will help.

As for my Clan Boss team, it's:

Kael

Drath

Apothecary

Frozen Banshee (40 only)

Outlaw Monk 40

I do between 4M and 6M.

I have a bunch of Clan Boss specialists (Yaga, Erinyes, Aothar, Aox, Coffin Smasher, Crypt-King, Anax), but none has topped my Outlaw Monk, who does around 1M each run.

I don't think I can improve it right now...