Overcome a 500 resource disadvantage in a match of Arathi Basin and claim victory.
You need a strong and obedient team for this achievement, but it's not terribly difficult.
There are many ways of doing this achievement, but the easiest is to allow horde to gain 4 bases, while keeping the 5th base neutral. You want this so that they'll get a lead of 500-0, which compared to for example 700-200 gives you more room to catch up before the opposing team wins. This also makes it possibly to do this achievement without ever controlling more than 3 bases.
As alliance defend stables to prevent it from being capped, Note, you can't cap it yourself. After the opposing team has capped farm, GM, BS and LM, you need to retain control of your stables, while also getting control of two of GM, LM and BS. 3 bases are sufficient to win when horde has a 500-0 lead. When horde has 460 points, it's safe to cap the 3 bases.
If you can't cap all 3 bases immideately, you will need to get a 4th base for a short while, as there is little room for error.
This strategy is ofc the same for horde players, just switch stables with farm. I prefer LM over GM in both cases due to the visibility from LM.