This secret achievement is called "Distance is the Best Defense". You get it by winning a battle with only Archers (and your General).
There are two ways of doing this;
First one, I had a save game later on in the English Campaign (after battle 15) where I had a full and diverse army, and more than 3000 gold. For this achievement I disbanded all units other than my Archers and my General. Disbanding units will give you a decent return on yourt previous investments.
Then I recruited 'Peasant Archers. I gave them new Armour so they became 'Archers', and a Falchion Sword. In the scenario I played, there was only room for 4+4+4 (-1 General) = 11 archer units so max out your archers to 11 units (if you play the same scenario...)
These archers hardly used their swords because the ranged attack killed 90% of the enemies before they reached my Archers.
I started the battle with 3 Longbow Archer units (levels 12 to 15) and 2 Handgun Archer units (levels 13 and 14) and about 7 or 8 fresh Archers (level 1). My general was NOT mounted.
I played the side mission where you have to loot two French carts. The enemy will run up to your army from the north and will all be shot down with arrows. I happened to have "Prevailing Winds" and "Starvation" in my Battle Cards, they really help here as well. Move up to the north to get the first cart. Hurry, because sometime the cart will start driving off and I think you do not actually WIN the scenario if you get the carts too late.
While one unit gets the cart, the others line up IN THE NORTH to move west.
I hugged the edges of the map to move all the way above the enemies lines in the west.
The English Longbow archers have a longer range than the French Crossbows, so use the Longbows to get the enemies in the West moving towards you (they'll start moving north now). Watch out for the cannon halfway the French lines, don't get in front of it. Move to the cart before it start leaving the battlefield and your achievement is here (in the next totals screen)
--=[ So your army starts southeast, moves north (all the way up), moves west and moves south. This way the French army has terrible position in the West (they expect you to come from the East, not the North) and your Archers will have a great time shooting down the units that move up North towards your army :) ]=--
I won this achievement this way the second time I tried. The first time, my General was MOUNTED. Maybe, just maybe I did not pay close enough attention and missed some other unit.. anyway, a dismounted general and only archers works for sure.
The other, easy way of winning with just archers is to restart the english campaign, win the first battle and the replay the side mission till you get enough money for archers, buy them, replay mission, buy archers etc.
Archers are gold, they level up really quickly because they get lots of kills in this scenario. There's no need to upgrade them, they'll shoot anyway.
Once you have 8 archer units, save your game and then lose the two infrantry units you got from the beginning.
Take out the advanced french crossbow unit and then retreat and let the french advance. Put your archers in defensive positions (blue shield) and just sit back and wait. If the french cavalry reaches your lines, switch a couple of archers to the red shield and flank them as well. Easy as pie this one.
Good luck!