Defeat the trio of Crown Chemical Co. apothecaries in Shadowfang Keep.
Note: The following guide assumes that no one is crowd-controlling Apothecary Baxter or Apothecary Frye. At this late point in the expansion, nearly all PUGs will out-gear the encounter. If your character is weak, it's safe to assume that other group members will be able to carry you.
First, go to the table with the neutralizers. (It's on the left after stepping off the stairs and it sparkles. Can't miss it.) Every member of the group should pick up Perfume Neutralizer and pick up Cologne Neutralizer. Everyone should start out with using the perfume neutralizer.
Also, this may seem a small thing but kill the horses in the nearby stable if you think someone may aggro them while fighting the apothecaries. Their ability to stun players is annoying.
Speak with Apothecary Hummel to initiate the encounter. The enemy NPCs will run to their starting positions and Hummel will emote a sentence or two.
Apothecary Hummel is the first enemy to become active. The tank should immediately taunt him, keeping him pointed away from the group to avoid splashing anyone else with Hummel's Alluring Perfume Spray (because there is always someone who wears the neutralizer). Anyone who has Bloodlust or similar should use it now (unless requested otherwise). DPS should use their DPS cool-downs at this time, too. Burn Hummel as much as possible before the next enemy becomes active. Avoid Chain Reaction by running at least 5 yards away from him.
Apothecary Baxter joins the fight 10 seconds after Hummel enters combat. There are two options here: the tank taunts Baxter and must be healed through his Irresistible Cologne Spray; or one of the DPS switches to Cologne Neutralizer and tanks Baxter. The melee damage by Baxter (and Hummel) is practically negligible in order to balance out their potent channeled attacks, so the second option is quite viable. Baxter will also sometimes channel Chain Reaction, so run out of its range.
Once Hummel is dead, the tank and mDPS should buff themselves with the Cologne Neutralizer. The healer and the rDPS may choose to keep the perfume immunity or switch to cologne immunity.
Apothecary Frye joins the fight 10 seconds after Baxter (20 seconds after Hummel). Frye cannot be tanked because he does not have an aggro table. Frye will run around throwing bottles of perfume (purple-pink) and cologne (green-yellow) on the ground where a player is standing. Players need to take care not to stand in the wrong 'puddle' or they will take steady damage. Players should focus on slaying Hummel and Baxter first, but cleave damage and DoTs are okay. Frye can be stunned, snared, rooted, Death Grip-ed or polymorphed. The stun is especially useful so that melee aren't chasing the little bugger everywhere when it's his turn to die.