Deadliest Warrior Battlegrounds

You need to Level up the Spartan 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.

I play a lot of FFA mode and average around 8000 xp per match. There are quicker ways to rank up but this is for sure funner and more bearable. Kills without dying gives you the kill streak ranks as follows:
Dominator: 0 bonus points (3 consecutive kills)
Unstoppable: 25 bonus points per kill at and after this streak each time you kill someone untilyou die. (5 consecutive kills)
Legendary: 50 bonus points per kill at and after this streak each time you kill someone until you die. (7 consecutive kills)
Godlike: 75 bonus points per kill at and after this streak each time you kill someone until you die. (10 consecutive kills)
Rampage: 100 bonus points per kill at and after this streak each time you kill someone until you die. (15 consecutive kills)
Also: Decapitions yield 25 bonus points per kill.
Head explosions yield 25 bonus points per kill.
Head shot kills with ranged weapons earn 25 bonus points per kill.
Victory bonus yields 500 points at end of match.