I think that if you are doing the challenges and your the one getting the most exp... then it should count towards passing a challenge. I played through all the challenges with team players but at the end it only counts when you do it solo...

Well why would they give you credit if you needed help and couldn't accomplish them yourself?

Not trying to be mean, I'm sorry D:

Here is a great way to beat the challenges: Start a new character and do each one up to 2 stars (If you want the attachments and stuff) from the beginning... And do them in order :3
There, you'll have them gotten easily

I agree some of those challenges are really hard to solo. I read somewhere it should give you credit for completion on multiplayer and then I tried it only to find it was false.

I honestly see no reason why the challenges should not give a player credit for completing them with others. If they were really meant to be played solo then why offer the choice to play them with others? Besides, we all know that the AI is crazy hard on defense while your teammates are lackluster at best. Add in the way the AI scales to your rank and the only plausible way to win is by deranking your character.