[Gameplay] [Co-Op] [Achievements] Guest Was Given "Challenge Accepted" Achievement With Host Set To Normal

Steps to reproduce:
1. Start the game on Hard difficulty as the host.
2. Have a guest join with Hard set client-side.
3. Have the host set the difficulty to Normal right before the boss Zhamla Meer.
4. Beat the game and observe that guest earned "Challenge Accepted" with the host set to Normal difficulty.

Seems like a simple fix, just modify the achievement to be based on the Host's difficulty while in co-op.

(And for the record, I wanted to finish on Hard and still will some day, my bro the host was getting frustrated. :p)
 
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