It seems like the Flying-type has been decided to be a type which always is paired up being that, so far, no Pokemon is naturally a pure Flying-type. This is probably because Flying is looked to be more of a skill rather then a type (like Fire, Water, Grass, Electric) or attribute (like Bug, Dragon, Dark, Ghost). Then again, it could be said Poison-type is more of a skill then an actual type/attribute itself, it seems like Flying just didn't get the luck of the draw. It should also be noted that Flying is always a secondary type, never a pure or primary type.

However, under special conditions, a few Pokemon can become a pure Flying-type:

Porygon/Porygon2/Porygon-Z - Using Conversion or Conversion 2.
Kecleon - If hit by a Flying-type attack, its Ability Color Change will change it to a pure Flying-type for that battle.
Arceus - Attaching a Sky Plate to it. It is the only Pokemon that can be a pure Flying-type outside of battle.

Though one can consider a Normal/Flying-type to be the closest to a pure Flying-type, being that the only thing the Normal-type does is give it an Immunity to Ghost-type attacks and makes Fighting do normal damage.

With Drapion being part Dark-type, both moves will get STAB so taking that into account:

Night Slash - will do 105 damage with a higher chance to be a critical hit raising the damage to 157 (It'll be rounded down).
Crunch - will do 120 damage with a 10% chance to lower the defending Pokemon's Defense Stat meaning the more damage the Pokemon will receive as the battl goes on if Physical attacks are used against it.

Personally I would prefer Crunch, as the "higher critical hit percentage" Night Slash offers is 12.5% to the normal 6.25% for attacks without it. In addition, is Crunch does a critical hit, you'll be doing 180 damage (though it is less likely to happen then a 157 damaging Night Slash, but not by much). So I would advise to take Crunch as it has a solid chance of doing high damage without doing a critical hit.

Now that Platinum has been confirmed to be coming out in March 22, 2009. I would suggest waiting until then to buy a Generation IV game.

There are 3 wat to make a hit Super Effective against Sableye and Spiritomb, two involve an Attack combo and the last involves an Ability. However even after doing this only one type can do Super Effective damage, and that type is Fighting-type. Wait a second, being half Ghost-type, aren't Sableye and Spiritomb immuned to Fighting-types? Technically yes, but there are always exceptions to the rule:

Foresight - A well known attack of Ash's Noctowl, Foresight's effect is one of the vein of all Ghost Pokemon. Foresight allows Normal- and Fighting-type Pokemon to hit Ghost-type Pokemon. With the immunity to Fighting-type which the Ghost-type part of Sableye and Spiritomb gave them gone, Sableye's and Spiritomb's Dark-type will make any Fighting-type move used from then on Super Effective. And just incase the Ghost Pokemon's evasiveness was a problem, Foresight also lowers the evasiveness down to 0 meaning you'll hit the opponent mostly all the time unless the Attack you use has the chance to miss already.

Odor Sleuth - Guess what? It is the same above except it can be learned by different Pokemon and was introduced in Generation III (Foresight was introduced in Generation II).

Scrappy - An ability introduced in Generation IV, it has the same effect as Foresight and Oder Sleuth except you don't have to waste a turn to activate it (once a Ghost-type Pokemon is out the Ability will activate) and it doesn't decrease the opponent's evasiveness. The only problem is that Scrappy is only known by two Pokemon: Kangaskhan and Miltank, both of which could also have a chance of knowing a different Ability. However both Kangaskhan and Miltank can learn pretty good Fighting-type attacks so the only thing you won't be getting is a STAB (Same Type Attack Bonus).

Incase you're wondering, using any of the 3 above methods will also make Normal-type attacks do normal damage. And if you want to use a Psychic-attack on them, Miracle Eye does the same thing as Foresight and Odor Sleuth except it is used to make Psychic-type attacks do normal damage to Dark-types.

You can find Oddish on Route 229 and 230. They are common and around the low 20s so bring a Pokemon around that level.

but dude..you can only get to those routes after you've beaten the elite 4 right?and I want a good pure grass type before I start my battle with the elite 4:-s

gddamnit..should've wrote it down..I swear it was in the great marsh that I ran into one..is that even possible?

I want oddish cause I've got a sunstone and want to train it into a gloom and then evolve it into a bellossom.

Don't like roserade..plus i can't turn a roselia into a roserade because I'm playing pokemon diamond on my computer(as roselia only evolves into roserade when it's traded..right?)