Deadliest Warrior Battlegrounds

You must obtain Rank 25 for the Samurai class, which takes about 145,000XP. You should go for 100 kills first for the other achievements, which leaves you around Rank 10. That still leaves a lot of grinding for XP.

I am not sure if this is the fastest method, but I did "Hold the Banner" matches, which last about 6 minutes and end with 3000 points for you if you go straight for the banner and hold it for 5 minutes. If you do this on the level, "The Moor", there is a small rock formation you can jump on where the AI cannot reach you by the water on the left side as you face the water. Just idle on top until the match ends for a win and XP.

You need to Level up the Samurai class to rank 25 to get this achievement.
You should go get the 100 kills achievement for this class first in about two Free-For-All Rounds. After that you should be around rank 10.

The best way I found to level up was to do a private Plant the Banner match on the map Isle against one bot. Just sprint for the banner and capture it three times without fighting the bot. The bot wont be able to touch you. It will take around 2 minutes 30 seconds a match including the loading times. You will earn 2500xp a game. You can get an extra 250xp if you decide to get one kill on the bot for the first blood bonus but that can risk you dying which will make the match longer. I Just focused on capturing the banner. This should take around 2 hours to get to rank 25. Hope this helps.