You get a very good vampire set from login rewards the first few days. Hopefully you havent sold it.
These items are scripted to all have crit rate, speed and att% as I recall. Use those items on kael and he shines.
Preferably you could farm lifesteal gear in campaign for a hp% chest piece it will make wonders to his ability to stay alive.
If you managed to get some decent perception gear to suplement that is fantastic. But the two offpieces does not have to be in same set. Stats are more important.
You should aim for a minimum of the follwing.
24k HP
70% crit rate
140+ Accuracy
170 speed
But as someone else said. You really dont have to beat brutal. The most important is you can farm stage 12.3
After that you have to decide if you want to focus on arena or clan boss because the ideal kael build varies abit. Clan boss you need to focus more on accuracy, defense and hp. In arena focus is more on speed and crit damage and attack.