Ok ok i know i have sent a lot of these your way and i do apologize for doing so, but i picked up *Emerald* the other day, dreamt on a team to go for, and need your guy's help, once more.
Sounds pretty good, but you might want to think about switching out Slaking or Crobat for a water type. Handy to have against ground/fire types and also most water types can learn Ice Beam, which is incredibly handy against Elite Four Lance, who is all dragon types.
Emerald is one of my favorites. Have fun! :D
There is ne'er a heart so pure than one who's love has been lost
From experience with Emerald I can say that Nuzleaf/Seedot are very VERY rare encounters and in my opinion not worth it. For one, the strongest dark move you get is faint attack and the strongest grass move, Giga Drain (meaning you need a TM). Outside of this, it has really bad coverage options otherwise and will end up being more of a burden
Ralts is a horror to train but Gardevoir is one of the best Pokemon in Emerald's "story" mode. Psychic + Thunderbolt hits basically everything and coming off that special attack stat, it picks up OHKOs. Nice defenses as well
To be honest, putting Slaking in on this team is going to pretty much force you to have a playthrough full of grinding. Stand alone, Ralts and Slakoth NEED the EXP Share to gain levels meaning a majority of the time you won't be using them and they'll become more burden than help. But if you wanna take this on yourself then sure. Slaking hits very hard with Hyper Beam and has wide coverage options. Its ability is horrible though and I feel that its prevolution Vigoroth puts in enough work
Crobat, again, is a ***** to train. Zubat itself doesn't have a very wide movepool and it doesn't get better upon each evolution. Literally you'll be forced to run Sludge Bomb, Wing Attack and two other moves. Its a good Pokemon in story mode but you have to be willing to invest time into training it