Agreed with Trips and Kramas. The difference between a 5* and 6* is enormous. I would be spending my time in Brutal 12-3 getting food for Geomancer, Deacon, Uugo, Fahrakin and probably Athel too, since she should be your campaign farmer.
If you feel the need to try Fire Knight 13, I would probably go Deacon, Geo, Athel, Fahrakin and I suppose Aleks, with what you have available now.
Also when posting roster screenshots it's easier to do it in this format, so we can see everything you have available in a convenient fashion:
Thanks a lot for replys, very useful
Aleksandr can farm very Good în 12.3. I Will go for Deacon as my net 6*.
I really need to clear stage 13 so I can get The sacred shard from dungeon challenge.
Thanks again!
Remember, this game is a marathon, not a sprint. That sacred shard isn't going anywhere. Whether you get it today or in a month from now, it'll still be yours in the end.