Medics are always in high demand for obvious reasons. Guardians are needed as they can take some damage, but their bullets stagger the bugs. Explosives work well with Guardians because the bugs get clumped up and easy for big boom. Snipers when utilized properly are worth their weight in gold as they have guns with the hit power and accuracy to take out Gunners, Infernos, and the butt blasters. Engineers are always needed like Medics because they can deploy free turrets and ammunition. Everyone has their roles to play