Alright, these challenges are starting to piss me off.
The game's controls are way too wonky and crazy and so is the gameplay. I just can't get used to it.
Yo Redneckromeo, I think you CAN get the unlocks EVEN if a friend helps. But here's the catch: YOU must be the one to complete ALL of the objectives YOURSELF. What your friends need to do is nothing but fight the enemy and fend them off. Basically they need to defend you and keep the enemy off of and away from the objectives.
At least that is why I think it did not unlock for you. I'll bet the same would happened to me if say, I did one objective, but my friend did the next two and so on and so forth. I believe it must be one person to complete all of the objectives to get the unlocks. So whoever does them all himself is probably the one who gets the unlocks in the Challenges.
I could be wrong as this is pure speculation, so don't quote me on that.
Just an idea/thought.
So speaking of that idea... Does anyone want to help me with that theory? I could use the help. You guys fend the enemy off, I do the objectives. And in return, I (And others) fend off the enemy while you complete all of your objectives for your challenge.
Anyone up for helping each other/boosting or whatever?