Left window make number 1 up to 15

Right window  1 up 20

I dont know what shell I keep.

and what heroes will be good together

Keep everything that is part of Rhazin and Relic Keeper fusion. Keep all epic champs. Keep all void. In your storage usefull are N3 (High Khatun) , N8 (Thenasil), N11 (Maneater) and N18 (Coffin Smasher).

That's really the shortest possible answer and it is by no means fully corect and accurate, because these champions still must be put in right team and right content, as well as some of the others may prove useful in certain cases, but  basically this is it.

*edit - well, there seems to be a shorter answer than mine., hehe :) 

Keep them all

1 Copy of each Rare, Epic, & Legendary

Yeah, Player J is right.