Your job as a merc (or any DPS for that matter) Is to keep pressure on the enemy team and backup your tanks ect. You're not going to be the glory hound and get all the attention and high scores, but what you will do is force the enemy back from your allied position, keep then thinking and moving
You fail because you don't know your role.
Everyone wants to be Manning but they forget he has 10 other people backing him up