A Poor Harvest

Invigorating Vapours is why you need this hero, it lasts for 2 rounds. which means every other round you can use something else. The rest of the skills are your choice. I used Festering Vapours for enemies on position 4 or Flashpowder for heavy hitting enemies.
Battle Ballad and Inspiring Tune are what you want. Battle Ballad lasts for 3 rounds so heal stress in between if needed, but you want to keep spamming Battle Ballad so your Man at Arms is buffed. The rest are your choice. I used Harvest only as a finisher and Finale for when the horses are suffling the party.
Divine Comfort is the only heal you have and need. Mainly picking skills that can be used from position 2. When you are not healing, stun with Dazzling Light, debuff with Illumination or finish near dying enemies with Mace Bash. Vestal does really well from position 2.
Defender, Retribution and Bolster are what you need, 4th option is up to you. I picked Crush but used it maybe twice. Defender and Retribution last for 2 rounds so you will be exchanging between them every other round and these will be your main damage dealing skills. You will be killing enemies with Riposte buffed from Jester's Battle Ballad. Use Defender on allies that are most in danger obviously. Bolster is for the fight with Miller. Using Shard Dust with allow you to use Bolster again and again. I only needed 2 for the achievement.

I think the primary attribute should be dodge then everything else. Jester benefits from extra stress healing and Vestal from extra healing. Basically use whatever you feel more comfortable. Jester's Battle Ballad will provide Man at Arms with accuracy and critical so you can skip those, plus speed for everybody. Other good options would be reduced stress, extra protection, extra HP.

The most important item here it the Shard Dust. I bought 4 and I used only 2 for Bolster twice against Miller and I got the achievement early in the fight. For the rest my advice is to buy all of them and don't hesitate to use them, you don't want DoT effects stacking on your heroes, you have only 1 heal and no skills to cure negative effects.

This tactic worked on Build #24839 and I earned the achievement on my second attempt with this set up. I didn't have any camping skills during the fight and 2 Bolsters were enough to trigger the achievement.