First thing's first, the achievement description is misleading because it says "In a Multiplayer match" implying only one game. This achievement is cumulative across all online modes and can be completed in as many matches as it takes you. Simply kill the required amount of enemies (up to a total of 2,000 kills for the highest achievement) and your achievements will unlock.
The best mode I have found to do this on is the 40 player mixed modes. Now I know what you're thinking, there's not as many enemies. But this is only slightly true, in 40 players the teams are mixed with a lot of added AI enemies taking it nearly as close in enemy count and with all of that fodder. The significance of the AI is that they have a lot less health and speed and they are not nearly as hard to fight against as real players.
AI will spawn in the same waves as the players and are very easy to kill as they can be killed in 1 hit by crossbow arrows and any heavy attack from any higher damage weapons. One of the best maps for this is Rudhelm as their are TONS of peasants to kill on top of the players and weak normal AI when you are the Mason team. You can often flank past your opponents and just 1 hit kill these guys with ease. This comes down to my class recommendations and combat tips...
Knight - Use the class you unlock at level 4 with the shield as this will make going against other players easier to block using
whilst still being able to dish out and take heavy damage including 1 hitting the AI
Vanguard - This class is strong right from the beginning because of his Battle Axe and how much range and damage they have. With the Vanguard's sheer speed to get into combat and how powerful the axe is you can basically sprint into combat and spam
or hold
for heavy attacks that you can activate before you even get close to your opponent. This will 1 hit kill most opponents if not 2 hits depending on the class.
Archer - For those who really enjoy ranged characters the Crossbowman is your best bet for easy kills which is unlocked at level 4 of the archer class and is significant because of its ability to 1 hit kill any AI enemies at any part of the body and the fact that it cant 1 hit kill a lot of enemies at full hp if you hit them in the head. If you're fast enough when getting charged you can hit
to swap to your secondary and begin fighting. The hand axe is best for this.
Combat tips:
1) Pushing your left thumbstick in any direction and pressing
will dash you the direction you pressed allowing you to flank opponents or dodge attacks
2) Tapping
will deflect incoming projectiles if your timing is correct including melee attacks
3) Holding
drains your stamina (yellow bar in the bottom left) which will make it to where you can't block anymore so try to time your blocks
4) While blocking you can counter enemy attacks by using the same attack as your opponent just before their attack hits (
for an overhead which does the most single target damage
for a stabbing attack and
for a swinging attack which can hit and hurt multiple opponents. Make sure to often switch between these attacks
5) Timing is key in this game, for quicker attacks especially with your swinging attack swing your screen in the direction you want it to hit just about when you press the attack button so it hits sooner. Similar with your other attacks
6) If you just blocked an attack or landed an attack on an opponent you have the timing advantage. If you ever find you don't have the timing advantage, dodge out of combat to reset the fight or successfully block an attack, do not try to attack first if they have the timing advantage it won't often hit first.
Have fun and good luck with your adventures, remember to fight with glory!