Pokémon X and Y Walkthrough, Pokémon Move Sets: Amoonguss
(Please note that the recommendations below are largely made for in-game play. An Amoonguss in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. If you've got some alternative suggestions, add them to the comments.)
Though it's not exactly the most attractive of pokémon, Amoonguss has earned its place as a force to be reckoned with. An HP sponge with health-restoring capabilities seeping out of every pore, Amoonguss is a fantastic choice for trainers who like to really irritate their opponents before taking them down. Effect Spore? Yes please.
Type: Grass / Poison
Amoonguss starts off as the tiny Foongus. All you need do is level it to 39 and it will mature into an Amoonguss.
Electric, Fairy, Fighting, Grass, Water.
Fire, Flying, Ice, Psychic.
Amoonguss is equal parts extremes and averages. Its HP is very high, while its Speed is very, very low. Everything else kind of falls into a middle range: Attack is good, Defense is meh, Special Attack is good, Special Defense is okay. Aside from the HP count, nothing terribly impressive - though Amoonguss' advantages fall more into its move pool and abilities anyway.
Effect Spore: When struck by a physical attack, the pokémon has a good chance (roughly one in three) of inflicting poison, sleep, or paralysis on its opponent. Though a little random and not always reliable, Effect Spore is great when it kicks in, whether or not your Amoonguss survives the attack.
Regenerator: The pokémon recovers one third of its HP upon switching out. A fantastic ability overall if you combine it with moves like Spore, and well worth the effort to obtain. Regenerator is a hidden ability.
Amoonguss is a bulky wall that's great at recovering its own HP, and should generally be used as such. Spore is Amoonguss' primary weapon, as it will always inflict sleep status upon the enemy, either giving your Amoonguss a turn or two to attack or giving you room to switch to something with better sweeping capabilities. Giga Drain is good for reabsorbing precious HP (and, in my opinion, preferable even to the stronger Energy Ball), Synthesis is a possibility if you anticipate a long battle, Clear Smog is really handy for debuffing booster pokémon like Aegislash, and Toxic is always useful if you like that sort of thing. Amoonguss is certainly the kind of pokémon that can survive a buffeting long enough for Toxic to become lethal, so long as you deploy it wisely.
EV / Super Training
Amoonguss is primarily a wallish pokémon, and so upping its defenses seems like a wise choice. Nevertheless, I'd advise training its HP instead. Amoonguss will already be receiving a healthy amount of HP each turn, and giving its HP as much of a boost as is allowable will raise its bulk to incredible heights. Beyond that, I'd recommend splitting the remainder between Defense and Special Defense to cover its weaknesses. If you really want to make it into a viable attacker, which it's decent at via Giga Drain and Sludge Bomb, throw a few points into Special Attack.
The only definite here? Never, ever bother with Speed. Amoonguss isn't going to outpace anything, save perhaps the occasional Avalugg. (And an Amoonguss against an Avalugg isn't a good idea anyway.)
Catching an Amoonguss
Compared to some of the other pokémon in this generation, snagging an Amoonguss is relatively straightforward. Track down a Foongus in the wild on one of the myriad of Routes where it appears (I recommend Route 20, where it shows up in hordes) and level it to 39 for your Amoonguss.