here's a couple tips for today's challenges:
throwing fireballs- vote for elite slayer. all the classes come equipped with two sticky grenades. and, if it's anything like the match i got it in, everyone will be throwing their grenades as well.
go right ahead folks- invasion matches offer a first kill in each round (i believe there's four or five rounds per match). on most maps the infected and survivors spawn relatively close to each other as well. just rush the other team and try to take someone out.