Win Arathi Basin by 50 points or less.
There are two fairly easy ways of doing this achievement, if you your team is strong enough and willing to follow instructions.
1. Cap farm/stables and LM, leave GM for the other team and fight on BS to prevent it from being captured by either side. Both teams will gain points equally fast, and your team can then cap BS with approx 1 min 20 sec remaining of the battle to get the slight lead allowing you to win.
2. Cap farm/stables and LM, as with the first strategy, but don't let the opposing team cap GM right away. They only need to be delayed by a few seconds, so your team gets points for LM before the opposing team gets points for GM. You will still need to keep BS from being captured. Capping LM before the other team caps GM gives you the slight lead that you'd get from BS in the 1st strategy, which removes the possibility of being unable to cap BS, which would result in a tie.
The 2nd strategy is risky, since you need to split your forces between 4 bases instead of 3, but in a different sense safer, since you won't risk getting unlucky with BS at the end.