Chivalry Medieval Warfare

Simple step-by-step:
-Have two or three accounts ready
-Select "Join game"
-Select "Team Deathmatch"
-Vote for Frigid, Battlegrounds Farm, or Dark Forest Valley; back out and and re-try if you don't get one
-Have the kill farmer on blue, kill feeder/s on red
-Blue picks Man-At-Arms with Broadsword, Vanguard with Greatsword, or Knight with Longsword - red picks Archer
-Blue runs to red's spawn, farms kill for 10 minutes
-Repeat

Change weapons mid-match and switch who is on blue team as appropriate.

This achievement unlocks at 100 kills with:
Man-At-Arms, using Broadsword's weapon class
Vanguard, using Greatsword's weapon class
Knight, using Longsword's weapon class
300 kills total.

In-depth walkthrough:
This guide is assuming you're kill boosting. Kill boosting requires two accounts - one for getting kills and one for being killed. Three accounts also work, by putting two people on the kill feeding team. You can play with other people, or double box for getting the necessary players.

Chivalry doesn't have anyone playing it, as of December 30th, 2021. If you do miraculously run into people, press Y on the game type select menu to change between the US West and Australia servers.

Use public matches. Private matches are known to only work for the Xbox 360 version. To join public matches, from the home screen pick the "Join game" option at the top. In the next menu, pick "Team Deathmatch."

Team Deathmatch games end after either 10 minutes, or 60 kills.

Have one account pick Team Deathmatch, then look at the three map options you're given on the right. You're looking for:

-Frigid
-Battlegrounds Farm
-Dark Forest Valley

Pick one if you see it. If two or three of these maps are available for voting:
-Frigid is better than Battlegrounds Farm
-Battlegrounds Farm is better than Dark Forest Valley

If one or more is available, vote for it then bring the other account/s in. If not then pause, pick "Leave game" at the bottom, and re-select Team Deathmatch, to get a new batch of maps.

I tested every map to find which are best for kill grinding. Frigid, Battlegrounds Farm, and Dark Forest Valley are much better than all of the other maps, from the perspective of blue getting kills inside red's spawn. These are the only maps where every red spawn point is within five seconds of each other, allowing for 35-40 kills per game, with one kill feeder.

All of the other maps have two to five groups of spawn areas, that are distant and/or obscured visually from each other. Dark Forest is a pretty distant 4th place, having two batches of spawns in complete view of each other - but they require a run across a bridge back and forth over and over. Try out Ruins if you want to see how bad the not-good maps can get.

Have the account getting kills on blue team (Agatha Knights). Pick Man-At-Arms with Broadsword, Vanguard with Greatsword, or Knight with Longsword. You can change weapons by pressing X while hovering over Man-At-Arms, Vanguard, or Knight.

Have the account/s feeding kills on red team (Mason Order) as Archers, because they have the least armor.

How to reach the red team spawns from the blue spawns:

Frigid:
Run straight, down the hill, going under the stone arch. Go passed the destroyed building and wood log trap, then up the next hill. The red team spawns in the clearing at the top.

Battlegrounds Farm:
Run a bit straight. Turn left at the river and follow it without crossing. Go to the left of the house ahead. The red team spawns in this open area beside the house.

Dark Forest Valley:
Run straight, down the small incline. At the fork in the road, go right. Run straight up to the gate. The red team spawns from this gate, and to the right along the small until the broken wagon.

Once the game is done repeat the prior steps to get back into a new game. Switch off between red and blue teams with your boosting partner, if you have one.

You'll usually finish a weapon's 100 kills mid-battle. Changing weapons mid-battle isn't intuitive; here are the steps:
-Pause
-Pick "Change class"
-Press X while hovering on the class you need next
-Pick the weapon you need next
-Press B
-Pick any different class
-Pause
-Pick "Change class"
-Pick the class you need next again
-Pause
-Press "Respawn"
-Wait 5 seconds

If you just pick a new weapon for the same class then respawn, you'll respawn without your weapon changing. You need to queue up the new weapon, pick a different class, then re-pick your wanted class now that it has the new weapon in the loadout, for the game to recognize your change, which you'll see play out in the bottom-left text box.

DISCLAIMER for posterity:
If and only if you see the Unobtainable flag on this achievement, disregard everything written above. An Unobtainable flag means that the Xbox One Chivalry matchmaking service no longer works, and it is now impossible to unlock this achievement, by any means.

The Xbox One Chivalry servers are working well as of December 30th, 2021. But as of this writing the game is delisted, Chivalry 2 was released on Xbox One June 8th, 2021, and nobody is playing Chivalry 1. I went 10 hours without seeing a single random person. The servers may not shut down for many years and/or Torn Banner Studios may release a thorough peer-to-peer service that keeps it alive for a long time to come. Just wanted to note this, in case you're wondering why the guide stops working at the "Select Join game" step.