Pumpkaboo is usually outclassed by Foongus because it doesn't have spore or regenerator, and Ferroseed because it has less resistances and no knock off or stealth rock. It's sometimes useful, but you should still consider the usually better options.
Pumpkaboo-super @ Eviolite
EVs: 204 HP / 196 Def / 4 SpA / 76 SpD / 28 Spe
- giga drain
- fire blast
Will-O-wisp threatens physical attackers. Giga drain covers water, ground, and rock, which are all pretty common types. Fire blast covers opponents that resist giga drain or threaten Pumpkaboo, such as Sandshrew, Spinarak, and Ferroseed. (fire is generally good because it makes things lose jobs :P) Synthesis heals HP. The EV spread lets it outspeed 10 speed Pokemon and wall some physical attackers, such as Mudbray.