You can't.
grass/posin
all of them are in emerald
you cant catch roselia in emerald, but I'm pretty sure you can catch it in Sapphire, so just trade it over
Roselia, Bellossom or Sceptile.
the Pokemon unobtainable are surskit, masquerain, zangoose, lunatone and Roselia
It is obtainable in RUBY AND SAPPHIRE but not EMERALD. You can find it on Route 117. You may need to use a level 10+ Pokemon to weaken it.
They are... Surskit, Masquerain, Meditite, Medicham, Roselia, Zangoose, and Lunatone
I am afraid you cannot find Roselia in leaf green.You have to trade it from other Pokemon games,but not Fire Red
You have to trade with Ruby or Sapphire to get a Roselia.
Budew is the pre-evolved form of Roselia. However, it is not introduced in Emerald, it is introduced in the 4th Generation games
You have to trade from... Ruby: Meditite, Medicham, Roselia, Zangoose Sapphire: Meditite, Medicham, Roselia, Lunatone Colosseum: Meditite, Medicham XD: Meditite, Medicham, Roselia, Zangoose, Lunatone These are only the missing Hoenn Pokemon.
Roselia can be found only in Ruby/Sapphire, on Route 117. Unfortunately you can't. Roselia can't be caught in the wild in Emerald. With that being said, it may be possible in a copy of Ruby or Sapphire, and you could trade it over. Otherwise, you're pretty much sunk.