Fruit of All Evil

This isn't covered in the achievement description, but it requires a minimum of 10 people.

As of 10.1.5 this achievement is soloable! Received in solo mythic run.

The name of this achievement gives a hint as to how to obtain Arcane Exposure for the sake of doing this.

Throughout the terrace leading up to High Botanist Tel'arn, there are various plants and bushes that spawn mysterious fruits that you can click on to eat which give you Arcane Exposure. This debuff is permanent so long as you do not die or leave the terrace (heading back inside towards the room you came in from will remove the debuff if gotten by mistake and you don't want to die). This debuff causes a number of weird effects to occur at random, which from searching through wowhead seem to be just three.

One makes you stop and throw up for a while, similar to what happens if you drink too much alcohol ingame. Another will suddenly knock you back a fair distance (and possibly others too, but unconfirmed), and the last one will make you see hallucinations, typically 1 HP targets that only you can see and need to kill to get them to stop bothering you (Do note that these put the entire raid in combat and no one else will be able to figure out why. If this happens for seemingly no reason in your raid and someone has the Arcane Exposure debuff, they likely have a hallucination spawned). In order to get this achievement, the entire raid must locate a fruit to eat to get the debuff and kill High Botanist Tel'arn without dying while dealing with the periodic effects of eating the fruit.