Can you fucksticks stop arguing about teamwork and how GK sucks, or how cool they are?
This isn't a GK problem, this isn't a teamwork problem, this isn't a problem with people not using the right equipment, this isn't a problem with cavalry, this is a team balance problem.
Stay the fuck on topic please (if you want any possibility of getting this problem resolved).
You are right there, it's a teamwork problem for a good part, but teamwork alone can't solve it. The game mode is the biggest problem.
Because even if one team holds formation, stays behind the shielders and the pikemen, the cavalry and archers will circle around them, and they will keep on losing players. The problem is the goal of killing the enemy team. If one team is holding formation (which means it consists mainly of infantry), then the other team, which could consist mainly of archers and cavalry, could still keep on attacking. They would be less effective, and perhaps they would take a few casualties, too, but if they are cautious they will still be able to fight to some extend, while the enemy "testudo" will only be able to react, wait and try to dodge. And as soon as the flag pops up, they would have to move there, which means a) they hold formation and come too late, or b) they break up formation and get horribly slaughtered.
As all classes are different in the way they are killing or can get killed, it is incredibly stupid to make killing the ultimative goal and expect everything to be balanced. Change battle mode into conquest mode, and see how archers and cavalry get demoted to mere support roles, instead of the initial source of momentum for a team.