My players encountered a Phantom Fungus tonight. The party is about 8th or 9th level.
The ranger in the party used a full round action to locate the invisible creature (he has an extremely high Spot check), then the sorceress used a fireball spell to kill it. Unfortunately, it didn't make any sound as it died, so they didn't realise.
The dwarven defender moved into range and hit it. (Squelch!)
The ranger fired three arrows at it. (Squelch! Squelch!)
The sorceress cast shield.
The dwarven defender hit it a couple of times. (Squelch! Squelch!)
The ranger fired three arrows at it. (Squelch! Squelch! Squelch!)
The sorceress readied a magic missile spell in case it appeared.
This repeated for a couple more rounds.
Finally, they realised it hadn't (a) attacked for several rounds and (b) hadn't moved. They stepped back and waited a few rounds for it to do something.
Eventually, the fried and totally dismembered corpse appeared.

Cheers!
The ranger in the party used a full round action to locate the invisible creature (he has an extremely high Spot check), then the sorceress used a fireball spell to kill it. Unfortunately, it didn't make any sound as it died, so they didn't realise.
The dwarven defender moved into range and hit it. (Squelch!)
The ranger fired three arrows at it. (Squelch! Squelch!)
The sorceress cast shield.
The dwarven defender hit it a couple of times. (Squelch! Squelch!)
The ranger fired three arrows at it. (Squelch! Squelch! Squelch!)
The sorceress readied a magic missile spell in case it appeared.
This repeated for a couple more rounds.
Finally, they realised it hadn't (a) attacked for several rounds and (b) hadn't moved. They stepped back and waited a few rounds for it to do something.
Eventually, the fried and totally dismembered corpse appeared.

Cheers!