His last one will be Samurott.
I suggest picking up Joltik in North Aventurine Woods, and train it in the Labyrinth for it's special attack EV's. It's a great fast Special attacker with the potential to be a physical attacker too. It will help you in the next two gyms quite well, and some of the gyms to come too.
Roserade isn't very good in this game, due to not learning any poison type moves on it's own and no access to Sludge Bomb TM.
(that isn't a Magneton, it's a Magnezone by the way)
I could tell you to backtrack the entire game and pick up some other Pokemon, as I personally have very little experience with your team as a team. I've used them individually in different teams but that was in prior generations, so I can't help you out much when it comes to movesets and stuff.
I believe it's still the swamp field, yes. But if your Pokemon outspeed his on default it really doesn't change much.
The field is actually to his disadvantage as moves such as Muddy Water, Surf, Mud Bomb, Mud Slap and Brine are doubled, some moves his team is quite weak to. Using Nature Power there also works quite well.