Uh. Which boss? 

Probably it's about the Clanboss.

1. Occult Brawler is a sure pick

2. Tayrel for Atk und Def debuff

3. Apothecary as a healer/ turnmeter increaser

4. Black Knight for Def buff und additional healing

5. Hard to say. Monster, Seeker and Lyssandra are all great champs, but maybe not ideal for Clanboss. If you use Black Knight, you allready have increased Def for your team, so Seekers Def-Buff is redundant. 

I think I would pick Lyssandra. Clanboss is immune to turnmeter-reduction, but her speed-aura and turnmeter-filling for your team still work and will give your champs more rounds to attack and heal.

Thanks!