I would use my favorite type: Grass
NPC's on my gym would be mostly Pokémon Rangers and Aroma Ladies, since they specialize in Grass-types.
To be honest, my favorite puzzles when it comes to gyms are quizzes, so I'd try and create some challenging quiz.
And so I would use pokémons such as: Exeggutor, Jumpluff, Ferrothorn, Breloom, I would probrably need Tangrowth to set up the sun, and of course my signature pokémon: Serperior would be my ace.
Then, as the challenger destroys me with Fire pokémon. Considering I use a sunny day team, it would make sense to give the challenger the TM for Solar Beam.