Fa-la-la-la-Ogri'la

Fa-la-la-la-Ogri'la is related to the seasonal world event, the Feast of Winter Veil. Completing this achievement is a criterion for the meta achievement Merrymaker.

Locations
Fresh Holly is acquired from a short list (Holly, Snowflakes, Mistletoe) when using a  emote on a  (not just any reveler), found in Eastern Kingdoms, Kalimdor, and Outland inns. You receive a 1 hour debuff when acquiring any of the items. In Dalaran, the revelers are in the faction-specific inns. You can also get Preserved Holly from Winter Veil quests.

The actual bombing run for Bomb Them Again! starts at the Skyguard Outpost in the west part of Blade's Edge Mountains.

Strategy
Simply be sure to use your holly at the time you turn in the quest (as a druid, for instance, you can do the quest in flight form, then sit on your mount and use the holly just before turning in the quest). See Bombing Run for help completing the quest.