An ol' man ever said to me to "Take advantage of the field!" or "Manipulate the field!"
Don't walk in a path your opponent made for you, walk by your own.
Did you know that Desert Field (the field of Florinia's gym) may increase Ground-type's Sp.Def by 50%?
You can use it as your own advantage. Eee.. maybe bringing a Ground-type is bad for this gym, but you can use Numel. It's ground and fire type.
Or, you can manipulate the field. Currently, the easiest way to manipulate or change to field is by using Castform.
Use Rain Dance then Sunny Day (or the opposite, but that will make Castform being a water type) will change the field to Rainbow Field for temporary, as long as the weather isn't faded.
There's many effect of Rainbow Field.
But the most important now, in that moment, Castform can use its Fire-typed Weather Ball with 50% boost.
Isn't that cool? facing grasses using 50%-boosted 100-powered fire-typed Weather Ball.
Anyway, you can look for Castform by trading it on a girl in Obsidia who is looking for a Furret. And Furret can be obtained by evolving a Sentret that you can get in Obsidia Park.
Castform is very useful for many battle in the future, until 4th or even 7th gym battle.
Also, keep collect the Field Note. Field is an important thing in Reborn battle. You can achieve your win against almost anything if you can make strategy of the field.
Good Luck and enjoy your game!