I dunno about anyone else, but even with the large number of weaknesses Grass has, I have a tendency to instantly think of Fire as my go-to counter against Grass. That trend almost holds up with Florinia-there's a little bit that it's not perfectly suited for-but it'll definitely make it easier.
Pansear can be found under the Opal Bridge during rain. It's best to get it early on-it's level 7 when you receive it-but if you're late to get it, grinding it higher shouldn't be too hard.
Numel would be a pretty great choice for this gym-It's part ground, so it doesn't have to worry about a sandstorm. Like Pansear, Numel can only be found during rain (In front of the Lower Peridot Train Station), and you'll also need some PokeSnax for it. It's level 15 when found, though, so that's much easier on you.
Litleo's kind of a matter of preference to get. After chasing it out of the Obsidia Alleyway and into the Underground Railnet, you'll need to take on an insanely powerful Klinklang to rescue it. That's not an impossible task, since you can OHKO it by collapsing the cave ceiling-but the only Pokémon that can learn a move that could do that is Numel, who would already fill a Fire-type slot on your team.
Far and away the easiest Fire type to get. It only costs 1000 coins, and you can find that many coins just by looking around the Arcade. It's at level 20, too, so there's not much levelling up required.
A Strategy for Florinia
The main benefit to packing a Fire-type against Florinia is taking down her Ferroseed. That sucker is only normally damaged by Bug, Flying, or Ice, and totally immune to Poison, but Fire will bring it down with 4x effectiveness, even better than Fighting could have down.
A decent Flying type has a number of advantages, too, though- It'll be immune to the Sand Tomb most of her 'mons will throw at you, and you'll get 4x on her Breloom.
Now, Cradily will likely be a bit harder. Its Smack Down will hurt both a Fire type and a Flying type, and it's lost most of its weaknesses-The only ones left are Bug and Ice, both of which are weak to Rock as well. But luckily, that Rock type also gives it two new weaknesses-Fighting and Steel. Three Fighting types can be found in the Obsidia Slums, and Klink can be found in the Underground Railnet from the entrance in the Obsidia Ward.