A bit boring maybe but I'd try a Normal type gym. I don't use Normal types usually, but I've been levelling random ones for a while. The team would maybe be something like this.
Raticate. Flame Wheel, Hyper Fang, Quick Attack, Crunch
Dunsparce. Aqua Tail, Rock Slide, Headbutt, Yawn
Porygon-Z. Psychic, Shadow Ball, Ice Beam, Zap Cannon
Blissey. Seismic Toss, Metronome, Softboiled, Hyper Beam
Loppuny. Sky Uppercut, Return, Bounce, Drain Punch
Snorlax. Selfdestruct, Body Slam, ThunderPunch, Outrage
Who wouldn't want to see a Snorlax explode?