While they can catch foes unawares, Amanita are not too dangerous in battle. They pose some threat when they team up with other creatures, but this doesn't usually happen very often. It was more common during the invasion of Smithy some years ago. As one may deduce from this, these creatures are normally wild, and therefore they don't normally have any affiliations.
Attack Pattern: When on the map screen, these little guys will often disguise themselves as regular mushrooms. You can avoid them by getting close, and then retreating to see if they "come to life".
In battle, Amanitas use plenty of regular attacks and occassionally attack with a spell called Spyrocyst that changes one member into a mushroom.
Basic Fighting Strategies: Regular attacks will work well - Mario can normally beat them with a timed-hit, and Mallow can also beat them with a timed-hit quite often. Fire is also effective, but costs an awful lot of FP, especially considering how weak they are defensively.
Advanced Strategies: It is probably wise to make Amanitas your first target in most battles, since you can usually beat them in one hit, and they can turn your party into mushrooms, effectively taking them out of the battle. If you've got trueform pins equipped, then leave them for last, as they are pretty weak.
According to Nintendo Power, Thunder should work well on them, but I didn't find this to be the case. Finally, blocking: Basically, they'll move towards you, then pause for a minute, then their "heads" pop up sort of like an umbrella. Press A just before the head pops up to block.
