Ah, Amoonguss... I love this thing. Ironically, despite being such a low-tier Pokemon, it's got a great place on Rain teams, due to being so stupidly bulky and annoying with its myriad of status and support moves. Here's my Rain Amoonguss set.
Amoonguss @ Black Sludge
Calm - Regenerator
252 Sp. Def, 252 HP, 4 Def
This thing is more of a utility set than a true wall, trolling my opponent with dual status and Clear Smog. Spore something, then Stun Spore as they switch out, since 99.9% of competitively competent players will switch their sleep bait out immediately. Giga Drain lets him do some minor damage while healing himself a little bit extra, but Regenerator and Black Sludge is really all he needs. Clear Smog shuts down attempts to set up on him.