You can find Duskull in Mt. Pyre. In Emerald, they are really quite rare, at a lowly 10% encounter rate. The only place you can find them inside Mt. Pyre is from floors 4-6. If you don't feel up to hunting them down, you can always trade from Ruby, since they are incredibly common in that one. :)
mt pyre
search on mount pyre
you evolve a duskull at lvl 37 one of the best ghost Pokemon on emerald
sky pillar or evolve a duskull
Duskull will start to evolve into Dusclops once it hits Level 37.
you evolve it from duskull or catch it in tower with the two orbs
It is very rare but you can catch it at mount Pyre.
Migrate from Ruby (or Sapphire/Emerald) or trade with Platinum
No.
Duskull is a Sinnoh pokemon as in pokemon platinum you get darkrai which is the evolution of duskull.
misdreavus you can get this from Pokemon leafgreen. Or shuppet/banette can learn it and even duskull/dusclops. gastly and all evolves can to
Duskull a Ghost type pokemon.