Don't be afraid to pick up a certain mon for a single Gym Battle and then box it forever - for example, Trubbish with Toxic Spikes can be useful against the second Gym Leader.
Reborn has common candies, that reduce a Pokémons level by one - don't be afraid to overlevel a Pokémon to evolve it, and then use common candies to lower it's level for a Gym Battle.
I'd also recommend getting a Noibat early on (you can get them around level 15, shortly after the first Gym). They have a good movepool, and have decent typing.
Get something with Detiny Bond, or a Ghost type with Curse (Duskull). They can help in defeating some strong singular opponents that guard event Pokémon (Like the Magnezone guarding Drought Vulpix).
Lastly, money is in short supply in this game. If you want to catch a certain Pokémon and you use too many Pokéballs - reset - those things don't grow on trees!
(Also, avoid the Game Corner - the prizes have been nerfed into uselessness)