I've searched for this using the nifty search feature, but didn't find anything. Also none of the Midsummer Fire Festival guides I've seen say anything about it, so I figured I'd bring it up.
http://www.wowhead.com/?achievement=388
To get this achievement, you have to kill 50 members of the opposing faction while you're inside one of your main cities. Most people go for this achievement whenever the opposing faction is trying to kill your factions leaders. For those of you who don't feel like waiting around for the Horde/Alliance to raid your city, here's a way you can get the achievement.
Your opposing faction will be trying to put out your cities bonfires, because of these quests.
Stealing Orgrimmar's Flame
Stealing Stormwind's Flame
Well, there's really a quest for EVERY main city, but these two are prone to getting hit by groups. So you get more kills
Anyways, just do your Midsummer Fire Festival quests near this bonfire. There's bound to be a horde or two (or alliance) trying to steal your flame. Once you're done with the quests, camping can get pretty boring, so you should probably talk in guild chat/trade while you wait. Since this is the first day of Midsummer though, the horde is coming in in waves, so you shouldn't have to wait too long. So yeah, just camp out by the bonfire and wait for some horde.
Hope this helped somebody!
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