The truth is Kael is very good in Lifesteal gear for Dungeon/ Clan Boss/Campaign
The problem is Lifesteal is just bad in Arena on a Speed nuking team.
However, you own Fu-Shan.
Fu-shan can do Godlike Damage.
You really don't need Kael in your Arena Team.
You can just keep Kael doing PVE stuff.
What I would do if I was you would be 2 things:
1 - I would properly build Fu-Shan has a Nuker.
2 - I would give Fu-Shan good set up.
To make Fu-Shan a Nuker, you want High ATK + 100% C.Rate + High C.Damage
I don't know your gear situation, but it doesn't matter because I like to do samples:
Sample 1 - 1k ATK + 100% C.Rate + 50% C.Damage
Sample 2 - 2k ATK + 100% C.Rate + 100% C.Damage
Sample 3 - 3k ATK + 100% C.Rate + 150% C.Damage
Sample 4 - 4k ATK + 100% C.Rate + 200% C.Damage
Sample 5 - 5k ATK + 100% C.Rate + 250% C.Damage
Sample 6 - 6k ATK + 100% C.Rate + 300% C.Damage
The reason I am showing you these samples is just to give you an idea.
Some people in this game are stronger vs. you.
Some people in this game are weaker vs. you.
The goal is to do best you can with what you have.
Than slowly improve on what you have over time.