I've created a Quest Mode group and played many adventures with them.

 

The characters are level 8, but the Party Tier is still level B and I only encounter cards of level B, C, and P.

 

How do I advance my party tier to 1 or more, so I can begin to encounter the higher level cards?

Looks like avg character lvl 1-9 = B, 10-19 = Deck 1 and 20-29 = Deck 2.

 

My guys just hit 10 and instead of saying party difficulty B it now says party difficulty 1. Now all that said, after I ran another mission I still just saw B level boons and I was already seeing deck 1 villains/henchmen long before that. So who knows. Plus considering everything else, it's likely to be buggy. But something like that appears to be the intent.