Next, let’s consider the Medic. Your team does have one, right? If not, get one of the 4 guys playing Sniper to switch. More on that later.
You have three sets of classes: offense, defense and support. Use your Engineers to lock down vulnerable areas in your base, and leave the offensive stuff to the classes meant for it. That means Heavies and Soldiers. They take point, which means soaking up damage, and the Medic’s place is at their side.