You levelled up some really... let's say "unusual" champs. I'm not even sure who they ARE. Is that a lvl 50 Interceptor between Athel and Lodric? I also recognize a lvl 50 Sorceress. You should really stop levelling trash!
You should lvl up your Stag Knight. He is a good champ. His A2 attacks all enemies and gives atk and def debuff at the same time. You don't have a good decrease def champ. Saito does this only single target, and Sorceress only 3 times random with a very low chance to land the debuff.
Another good champ is just sitting in your Vault: Reliquary Tender. She combines debuff cleansing (what is usefull in dungeons: poison from Spiderlings, freeze from Ice Golem, a bunch of debuffs from Dragon) with revive, what is nice to have if something went wrong, the revive saves the day.
How long do you play allready? You introduced yourself as a noob, so I hope you did not get High Khatun als log-in reward so far (day 30). I really hope you did not use her as food. High Khatun and Stag Knight together will help your Arena team a lot.
1. High Khatun in speed-gear --> speed buff and turnmeter filling
2. Stag Knight in accuracy gear --> AoE def debuff
3. Athel in damage gear --> Atk buff to herself, extraround, AoE attack
4. Saito in damage gear --> taking out any surviver