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).