Dec 25, 2021, 13:2112/25/21
Khatun next, then Sepulcher Sentinel IMO. You need to start working on a CB team, and she'll be very good for that.
As for your problem of Kael not being able to do 12-3 - that's because of your gear. Get him an amulet and a banner ASAP. Also - stop leveling 3* gear to 16 immediately. Find items from the shop that are 5*, even 5* rares will suffice for now. Does he have masteries, and are his skills fully booked? If not, you probably should work on those too.