As a well-oiled machine
Archers can be annoying, or just another notch on somebody's axe, and perhaps the 4th pick-up "Free special attack". A Heavy Knight can be as tough as nails in plate armor, or he could be a slow idiot for enemies to use as target practice. Men-At-Arms can cripple their enemies' morale with blinding insults and scathing retorts, or cripple their own team's morale by speeding off and being slaughtered.
The Knight team carries both sides of the coin in terms of class balance. Alone, an Archer, Heavy Knight, or Man-At-Arms have weaknesses that can be severely exploited, but together they are a deadly efficient war-machine. An Archer depends on a Knight for protection against enemies in melee range, the Knight relies on the Man-At-Arms' speed to nail enemies trying to kite him, and the Man-At-Arms requires an Archer to weaken enemies to a manageable level.
A Knight can act as a bodyguard for an Archer, while a Man-At-Arms hunts for enemies. The guard Knight follows Archers around and protects them from assaults and ambushes; the hunting Man-At-Arms flanks and finishes off weakened opponents, or flushes them towards Heavy Knights; and the Archer shoots enemies trying to get past his teammates. An effective Knight team finds the balance between Archers, Knights, and Men-At-Arms.
In specific situations, lots of Archers, Knights, or Men-At-Arms can prove effective. Tight indoor quarters? You want a lot of Knights. Constantly being given the slip by Skirmishers, Berserkers and the like? You need Men-At-Arms. Open areas with easily defensible elevation? Get lots of Archers. Volley!
When on a Holy Grail crusade, Knights should push to a position where their Archers can take advantage of their range with Heavy Knight protection. Knights can also send out Heavy Knights en-masse in close quarters, but they should be prepared for keg spam and shield/spear charges. With the Archer strategy, you should have at least one Heavy Knight guarding the Archers, and some Men-At-Arms roaming. During lulls in the combat, the Men-At-Arms should check around for Skirmishers lighting their kegs, Captains loading their guns, or Vikings trying to resupply.
When defending your wealth in booty, the Knight team should have Heavy Knights between the savages and the treasure. Archers should cover the open areas and chokepoints from a distance. Men-At-Arms should be in the middle of the defense, ready to help where needed. Knights should consider advancing on the enemy's base to more easily defend fewer choke points. Attacking Knights need to know when to siege and when to assault. Five Archers storming through a door are only going to feed the enemy special attacks to kill the Heavy Knights when they catch up. It is wise to have the Men-At-Arms and Archers regroup near the entrance to the enemy base, and let the Knights go first. In the assault, Archers should stay back and provide ranged support while the Knights and Men-At-Arms kill enemies and take treasure. Men-At-Arms should pass the treasure they've stolen to an Archer, then turn to fight or steal more. Heavy Knights can block doors with their shields if enemies try to stop the carrier.
Last Team Standing
When dealing wholesale slaughter, a Knight team should place Heavy Knights with shields in front to guard their weaker teammates, then advance cautiously to avoid being surprised by kegs and gunfire. Archers and Men-At-Arms should find Skirmishers trying to throw kegs and take them out before they have a chance. Heavy Knights should attempt to bash kegs away from their teammates if possible. Archers can suppress Vikings, and Heavy Knights can defend against and deter Pirates, allowing Men-At-Arms can cut them down.
When securing territory, Knights should approach from an angle that grants their Archers good line-of-sight, and preferably a chokepoint for Heavy Knights to fall back to in case of emergency. Once a territory has been secured, only a few Knights need to stay behind and defend it. The others can launch assaults on other territories.