Fungal Fury

To get the achievement for Fungal Frenzy, it's actually not that hard if you pay attention to what the mushrooms actually look like (they all say they're "sprouting crimson mushrooms", but they don't all look alike). The mushrooms' effects are NOT random; the debuffs depend on which of the 4 you click on.

I also posted the following info in the Fungal Frenzy achievement comments section.

Some details about the 4 different mushrooms (descriptions and screenshot links):
Poolstool (large red cap with faint blue splotches) can be found near the water's edge.
Poolstool screenshot

Shrinkshroom (hexagonal beige/pink cap with rips in the edges) can be found near rocks and other larger mushrooms.
Shrinkshroom screenshot

Shuffletruffle (blue cap with gold spots) can be found near rocks and other larger mushrooms.
Shuffletruffle screenshot

Trapcap (tiny round lavender cap with pink spots) can be found near the large mountain in the center. MUST be done last for the achievement, and will explode causing damage and knockback.
Trapcap screenshot

It seems the easiest to start by the water's edge (Poolstool), then move inward toward the central mountain and check the rock clusters on the way (Shrinkshroom and Shuffletruffle), finishing off at the mountain (Trapcap).

One of the Daily Therazane Quests that is unlocked upon rebuilding the World Pillar in Deepholm.

The Mushroom are small interact-able objects, more commonly found at the base of the red crystal formations sticking out of the ground, although they can still be found around the base of any terrain feature (Big mushroom, Rocky out-cropping etc etc) within the Crimson Expanse, in the eastern side of Deepholm.

When activated 1 of 3 things can occur.
1 - You gain a Shuffletruffle debuff, increasing your unmounted movement speed by 50% and reducing your attack speed by 30% for 2 minutes.

2 - You gain a Shrinkshroom debuff reducing your size by 50% and your running speed by 20% for 2 minutes.

3 - They explode, dealing 3570 resistable Fire damage and knocking you back and into the air, in the opposite direction you were facing. (So make sure your back is faced away from mobs and towards higher ground!)

The quickest way to do this is the mushrooms by the outer rock formations, they are in groups of 3 usually and very easy to spot because of the rocks, also they give speed and shrink buffs instead of exploding.

After 3 days doing this purely on the exploding ones it was handy to figure this out!