Amoonguss

This Pokeball 'shroom doesn't really have much going for it, unfortunately. It has low attack, an exceedingly common dual-typing, and no STAB quick move. It does have a really nice Dark Type move combo, but it's terribly outclassed in that regard. It's okay on defense, but is still easily bulldozed by many common gym cleaners.

Best PvE Offensive Moveset

• Astonish is just bad, making Feint Attack Amoonguss' Fast Move of choice.
• Foul Play is a good move, and it's the only available Charge Move that has synergy with Feint Attack.
• Grass Knot is stronger, and has better coverage than Sludge Bomb.
• Return* is worthless on offense

Best PvE Defensive Moveset

• Astonish is just bad, making Feint Attack Amoonguss' Fast Move of choice.
• Grass Knot has a high base power, decent coverage, and STAB, making it the move of choice.
• Sludge Bomb is another great choice, though it's a bit weaker and tends to be resisted by a lot of threats.
• Foul Play is a bit weak and gives redundant coverage, but it hits gym cleaners such as Metagross and Latios fairly hard.
• Return* is extremely hard to avoid, but it's too weak to be much of a threat.

Feint Attack has good neutral coverage, solid counter-coverage for Amoonguss against Psychic Types, and sports an average damage/energy generation profile, making it nicely rounded.

Amoonguss' main charged move is Grass Knot due to its decent power/energy cost, and good STAB coverage. The second move of choice is Foul Play. While a bit weaker than Grass Knot, especially due to the lack of STAB, it is Amoonguss' cheapest move, and allows it to put large dents in the aforementioned Psychic Types. Just note that Amoonguss is generally eaten alive by Confusion users, so be careful when using its coverage!

Sludge Bomb is another STAB move that is definitely usable, but Amoonguss ends up similar to Venusaur with this move, which leaves it horribly outclassed.