This achievement requires you to play as the knight (fourth player from the left) and to get 300 kills evenly distributed across 3 weapons.
100 kills with the two-handed axe (first row when you edit the class,
):
100 kills with the two-handed blunt/mace (second row when you edit the class,
) :
100 kills with the two-handed bastard/sword (third row when you edit the class,
)):
There is a small community that still plays almost all the game modes at the time of writing this so realistically you wouldn't need to boost. Just keep joining games and slashing away. The best game mode is definitely free-for-all since you have the greatest chance for kills and its very fast paced.
If you are going to boost this achievement then you can set up a private match, team deathmatch on Dark Forest, and invite your partner. The person losing should be on the Masons. Only one other person is needed to start the game. You'll pretty much have 15 mins to spawn camp your opponent to rack up roughly 50 kills, depending on speed and spawns. Unfortunately at the end of the match the game auto cycles to a new map so you'll have to quit out and host another match.