Let's say for instance you have a party consisting of a Cleric, Thief, and Champion. During your delve you flip a 3rd dragon card which is the Undead Dragon (weak vs Cleric).
Are these valid options for dealing with the dragon:
A) Use Cleric + Thief + Champion to defeat the dragon
(to score 3 XP points instead of just using Cleric + other)
Yes, you can do this.
B) Use Champion + Thief to defeat the dragon
(Champion acts as a Cleric for the weakness. Not sure if this work since some dragons have a Champion symbol specifically and Champions say they can defeat all MONSTERS of a single type while the dragon isn't referenced as a 'Monster')
No, this is not legal. Champions are good at fighting monsters (goblins, skeletons, and oozes), but Champions are only good at fighting the specific dragon that says it's weak vs Champions (I forget the name of it offhand... Elder Dragon maybe?)