I don't think you guys understand that this guy is a wall, not a slow sweeper. Nevertheless, here's a double battle moveset for Amoonguss.
EVs: 128 Def, 128 Sp. Def, 252 HP
Protect, Substitute, or Double Team
You can't go wrong with Spore. It has a 100% (or 99.6% to be exact) chance of making the foe fall to sleep. Rage Powder makes the opponents hit amoonguss, which is great if you have a tank or sweeper ally (especially slow ones like reuniclus). Giga Drain is a great move in inflicting a decent amount of damage and for gaining some HP. Protect would be good for gaining HP from black sludge while not getting hit from the opponent. Substitute would be great for a wall (period), and Double Team so that nobody hits you when you use Rage Powder. Overall, this guy would be great in double battles.
I personally use an amoonguss with this moveset and it works very well. With all that HP and defenses, it seems to me that it is a no-brainer that this guy should be used as a wall (but for wifi double battles ONLY), and even though amoonguss doesn't have very good moves, these (in my opinion) are the best moves that an amoonguss can have. I know there are going to be people that disagree with me on this one, but I use this moveset myself and it works great. Also, I would prefer Protect over Substitute and Double Team.