Ithos is the Best AOE Damage Dealer you listed out of all the heroes you said.
You should go with Ithos as your Nuker.
I would book Ithos.
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Abbess is the AOE Def Down hero you listed.
You said she isn't booked which means she is useless.
You should remove Abbess, unless your planning to book her to 100% Chance.
I don't know how many Legendary Books you own.
Do you have enough Legendary Books to book both Ithos + Abbess?
If you do, You can do it.
If you don't, I would remove Abbess as soon as possible.
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You did say you owned Stag Knight.
Stag Knight is another great option for AOE Def Down.
I don't know how many Epic Books you own.
Do you have enough Epic Books to book Stag Knight?
If you do, You can do it.
If you don't, I wouldn't use Stag Knight.
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You did say you owned Warmaiden.
Warmaiden is the cheapest option.
You can get her skills maxed out by farming copies of her in Campaign Chapter 9.
There is no shame in using Warmaiden.
Warmaiden is an Amazing Champion.
I even use Warmaiden for my own teams.
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Based on what you said, Your other 2 heroes are fine.
High Khatun + Gorgorab should remain on your team.
They are great hero's.
Based on all of the above information, your new team can be the following:
Leader: Gorgorab - Speed Aura 23%
1st move: High Khatun ------------------------> Turn Meter Fill + Speed Buff
2nd move: Gorgorab ---------------------------> Turn Meter Fill + Increase Attack
3rd move: Abbess/Stag Knight/Warmaiden ---> AOE Def Down
4th move: Ithos ---------------------------------> AOE Nuke
What I am going to tell you next is very important.
Don't beat yourself up over the AOE Def Down hero!
Why? Because 1 day you are going to do Faction Wars!
Guess What?
Every Faction Team you make is going to need an AOE Def Down hero.
Stag Knight - Banner Lords Faction
Abbess - Sacred Order Faction
Warmaiden - Barbarian Faction
Do you see what I am saying?
There is no point in you beating yourself up on who to use because 1 day your going to have to use all 3.
Just use the AOE Def Down hero which you can fully skill up in the short term.