Will M. Baker
If I want to use both physical and magical attacks do I need to discard 2 XP total?
I can see how your friend might read it that way, but 2 XP must be discarded to void both traits (the traits are evaluated independently when you select the monster to fight).
I tried to find an older version of the card online to see if it was clearer but couldn't find one.
The Doomknight•Death monster group is new to Into the Abyss.