If you're talking about endgame I assume you're talking about clan boss. I can only share how I go about it but In a nutshell, personally, I only invested in champions that I was sure could do (or could eventually do) Ultra Nightmare. If a champion is good but not "ultra nightmare" good, they get to level 60 but never fully booked, then used temporarily. Given the extreme scarcity of legendary books, whoever gets booked should be able to carry the whole account to the end game because Clan Boss is the ONLY reliable way to get a chance at more skill books. 

So for me it's simply determining roles and choosing the end game champion that fit into them:

- ATK down   

- Damage dealer (poison or burn debuff), 

- counter attack 

- Healer/cleanser/support

After choosing champs for each of the roles, it's just a matter of 3 questions:

-Do they have enough speed?

-Do they have enough sustain?

-Do they have enough ACC?

If any of my members lack any of the items above, I just go back to the dungeons till my team is properly geared up. Good luck!