possibly a Grass gym... with only 2 pokemon so the can be a higher level, or a mixed gym with 5-6. i think natures are randomly generated for trainers, so i wont bother with those.
Lillygant/Own Tempo - Sleep Powder, Quiver Dance, Petal Dance, Synthesis (get them to sleep, attack/recover/boost-stats as neccesary)
Breloom/Poison Heal (holding Toxic Orb) - Spore, Focus Punch, Facade, Leech Seed(?) (get them to sleep, facade the flying type they send out, focus punch everything else they switch out to, but put everything to sleep basically. use leech seed in a situation where focus punch isnt usable but facade doesnt work... such as an Insomniac Ghost-Type)
in this case, my gym would be a bog, with ponds and mushrooms. trainers could be hidden in the ponds (which dont require Surf) or in bushes, or posing as mushrooms. given that bogs and marshes are very volitile, this gym would be a basic maze in structure.
Trainers use various bog-based pokemon, such as the breloom i use, and maybe even stretching to that stunfisk, quagsire, etc.
that does open up my types a little, so...
Open with:
Whiscash - Earthquake, Surf, Ice Beam, Zen Headbutt/Future Sight/Other (catch out fire and flying type type openers, a grass type is possibly still beatable if the worst happens.)
Forretress - Spikes, Stealth Rock, Explosion, Ohter (probably Bug Bite) (and i think you can see where im going with that one)
lillygant, breloom...
pushing it a little, perhaps Poliwrath/Damp - Hypnosis, Focus Punch, Waterfall, Circle Throw (to force spikes damage)
this might look better in the Elite Four, rather than a gym. the pokemon are roughly 45-50, and arent too specific on type, although the new elite four are dead-set on a singular type now. in which case, my room would be the marshy bog, and my spiral staircase could be one of those wooden cage lifts, lifting them past the nice purple poison-y smog cloud and too my platform on top of the central tree.
as long as these pokemon are quick enough, i think theyll give anyone a hard time.