Against the Odds

The Tenacity buff is given to the faction with the fewest members in the Wintergrasp zone during a PvP battle for control of the Titan Relic.

This achievement can be earned by winning the battle while being (sufficiently) outnumbered.

It is known that the Tenacity buff is only applied to characters level 70 or above, so level 70 is a minimum for gaining this achievement.

Note
As of the release of Patch 3.2.0, either the patch notes were wrong, or there was a bug with this achievement, because it was not correctly converted to a Feat of Strength. It does not appear to be in the game at all.