The leach debuff is a really nice addition that quite frequently allows me to attack with Zelotah and Apothecary instead of using their healing abilites. This allows the other champion (which would have been stunned otherwise) to attack this round and thus deal damage. Now for the survivability: I put Foli as the leader, so that the clan boss will focus him with his stun attack instead of focusing someone else. In conclusion, all champions deal more damage thanks to Foli, but this extra damage does not get accounted to his damage dealt (60% Deff reduction roughly increases a GS/WM proc from 47k to 65k ish). Now Foli comes with a reliable Deff debuff, which increases the damage output from ALL Giant Slayer / Warmaster procs. As stated above, clan boss fights are all about surviving as long as possible and landing as many hits as possible.

Imho comparing the raw damage ouput does not suffice to determine which champion is the better one. Given that both champions survive for the same amount of time, Kael has a much better chance of dealing more damage. Since Kael has Warmaster AND Poison debuffs, he posseses one more source of damage than Foli does (since he has only Giant Slayer).

This is why you aim for high speed stats and try to survive longer, so you get in more hits and thus procc Giant Slayer / Warmaster / Poison more often. Normal damage from auto hits is almost neglectable. The main damage source at the clan boss comes from Giant Slayer / Warmaster and Skills that scale with enemy max HP, such as poison debuffs. / EDIT: To all those pointing out that Kael outdamaged Foli:

Raid shadow legends foli