The Man-at-Arms is a powerful defensive support. Easy staples include Bolster to raise the party's DODGE and Stress resistance, Bellow for a party-wide Stress-inducing DMG debuff that doesn't provoke Ripostes, and Defender to protect anyone being threatened. Since he contributes heavily to survivability, he's an easy inclusion to most parties, especially Stress teams. The last skill to use is based on your preference - Retribution lets him contribute to damage with Riposte and Rampart can shuffle, reposition, and Daze. Command gets used by fully supportive Men-at-Arms to boost the entire party, and often warrants a combo with Defender for maximum mileage.
Common Trinkets often seen on the Man-at-Arms are mainly to increase his survival and Stress infliction, since it's not unusual to see a Man-at-Arms spamming Bellow during turns where he doesn't need to Guard or attack. The Carnival Masque and Pitfighter's Helm are commonly used, while builds focused on survival prefer the Gladiator Helmet or Bloodstained Gambeson. When available, the Veteran Gauntlet and Protector's Kite are great Trinkets to use - the Gauntlet lets him occasionally go on the offensive with bonus Dazes while the Protector's Kite raises his durability and Guard duration. The Shield Spike will take a while to unlock, but will make his Riposte a powerful tool once it boils down to the last man standing.
Since he's so easy to use, it's not unusual to see your opponent run a Man-at-Arms of their own. There's a high chance that it's a Stress team, which will lead to a very long fight as he spams Bellow until your team can't hit very hard. Since Bellow can be used from anywhere, shuffling him will not disable him, and his high Stun resistance makes Dazing or Stunning very difficult for teams without Stun-increasing Trinkets.
There are two ways to counter a Man-at-Arms: Kill him first, or kill everyone around him. Since enemy Men-at-Arms usually take their first turn to Bolster the party, the first turn is your opportunity to try and kill him as fast as possible before the DMG debuffs set in. A Mark-based party can put on enough pressure that the opponent has no choice but to take other characters' turns to try and save the Man-at-Arms before he dies. A DoT-based party can largely ignore his DMG debuffs and high PROT to beat the opponent at the grinding game, but be aware of enemy ailment cures that can offset your efforts. When he goes low on health, Stun or Daze the enemy healers to maximize the number of opportunities you have to land the Deathblow.
If you're going to kill everyone around him, the act of focusing down an enemy unit will usually prompt the Man-at-Arms to Guard them instead of the usual Bolster or Bellow. While you can proceed to focus down someone else as he can't Guard two units at once, it's best that you have the ability to bypass and break Guards. A Shieldbreaker or Abomination can easily disrupt his Guard attempts, while a Jester can assassinate the target with Dirk Stab if they're already at Death's Door. An Arbalest or Musketeer can bypass Guards to finish off a Marked enemy, but they require the Stabilizing Tiller or Iron Sights respectively to do that job. Also be aware if the Man-at-Arms has access to Riposte, because when he's down to the last man standing he will get a lot of mileage from Retribution when you try to land the killing blow.
If the DMG debuffs are getting to you, counter with the Arbalest'sIlluminating Flare or the Crusader'sRally to the Flame. Do this as the first action of the turn or after the enemy Man-at-Arms has acted so that you can have as much breathing room as possible to pile on damage.