After you catch giritina youll come to spring field and it will be there in the grass!
You can catch Duskull in Pokemon Platinum by going to the Lost Tower that is near Solaceon Town however you can only find it at night.
Turnback cave.
Pokeradar on Route 224. It's easier just to catch Duskull in the Lost Tower at Night in Platinum. (Duskull don't appear there in D/P, only Platinum.)
Route 224 Poke Radar
Migrate from Ruby (or Sapphire/Emerald) or trade with Platinum
Duskull is a Sinnoh pokemon as in pokemon platinum you get darkrai which is the evolution of duskull.
the route between solaceon town hearthome city(at night)
Duskull evolves into Dusclops at level 37.
After beating the Elite Four, somewhere in the Survival Area at night. Or catch a Dusclops at Sendoff Spring and breed it.
lost tower at night
u cant give it owt because duskull evolves at lv 37
It is very rare but you can catch it at mount Pyre.