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.