marshtomp
Any Pokemon with ice or electric moves
the closest you can be to a gym leader in this game is setting up a secret base with your best Pokemon then mixing records
Winonna is the 6th gym leader and she uses flying type Pokemon. I'd suggest using a Pokemon with electric or flying type attacks.
doesn't matter. If you train your Pokemon to an high enough level you will be able to defeat the gym leader. I think the best STARTER Pokemon is fire-type.
Ditto.
rayquaza
Use water pokemon, grass types or bug types because both Solrock and Lunatone are weak to these types
Pokemon snap or Pokemon emerald
you evolve a duskull at lvl 37 one of the best ghost Pokemon on emerald
torchic
yes he is probably the best pokemon in pokemon emerald
the best Pokemon region is hoenn { ruby,Sapphire,emerald }