I’ve now started working on the Shaman for the same achievement.  He has a few of them, including Hallow’s End and Brewfest, but he still needs 4 or 5 more of them.

Here are the achievements for Hallow’s End

* Trick or Treat! – done at any Innkeeper in the game
* Out With It – eat so much candy that you puke.  this is easiest done with the sweets you get from the Headless Horseman quest after killing him.  They look like pumpkins.
* Bring Me The Head of… Oh Wait – Kill the Headless Horseman (5 man quest)
* The Savior of Hallow’s End – Put out fires in Brill, Razor Hill or Falconwing Square when the Headless Horseman attacks
* That Sparkling Smile – loot a toothpick from any Innkeeper by Trick-or-Treating.
* Rotten Hallow – Get 2 quests outside of Undercity on the road to Sepulcher.  Take those quests to Southshore, in Hillsbrad Foothills, turn one of them in inside the inn, and throw stink bombs in the center of town.
* G.N.E.R.D. Rage – kill 50 allies while having the GNERDS buff.  These are candy you get from the Innkeeper every hour.  This achievement is easy to get in Wintergrasp.
* Check Your Head – From the daily quests or from the Headless Horseman, you get pumpkins to throw at other players.  Do this for each class and you get the achievement.
* The Masquerade – This part is NOT needed for the Hallow’s End title or achievement.
* Sinister Calling – Loot a Squashling Pet and a Helm from the Headless Horseman OR from the Innkeeper Trick-or-Treat loot.
* Tricks and Treats of Azeroth – Go to all Inns in Eastern Kingdoms, Kalimdor and Outlands, getting candy from the big Pumpkin with the ! over it.  This gives good XP if you aren’t 80 and 6g63s if you are 80.