Re: Challenges to level up are pretty ridiculous.
I find in most games the team usually needs a little bit of "encouragement" to actually go for the objective. After someone starts it off, everyone tends to follow. Though it does get annoying.
The challenges really just take patience. I tend to only play a class when I know I can complete them in that game. If I spawn in as cactus, see no drones - go play another class. Sunflower was totally worth getting to level 10 though, that class unlock!