Shadow Gloom

Razor Leaf + Sludge Bomb and Petal Blizzard or Moonblast

Razor Leaf is very powerful with STAB, and should be taken advantage of whenever possible. Acid doesn't provide a large difference in energy generation and deals way less damage, so it's not as preferable. 

The poor energy generation from Gloom's fast moves results in infrequent use of its rather expensive charge moves. As a result, Sludge Bomb is used a majority of the time simply because Gloom would likely faint before getting enough energy for Petal Blizzard or Moonblast. In the rare cases where Gloom would endure a prolonged fight, access to two Sludge Bombs would pressure shields more than just using one expensive charge move.