Fun question: Have you guys tried a all pokemon type challenge at the gym.

I once only used my four 3k Dragonites (DB/DC * 1 + DB/DP * 3) to take down a Level 10 gym full of Dragonites and Snorlaxes. But three of the Snorlaxes were Lick + Earthquake and under 2600, so they weren't much of a trouble. So I reckon, if I had six 3k Dragonites, I could manage to take down any Level 10 gyms.

Before the CP adjustment, my friend tried a Wigglytuff army. It worked pretty well! They took down a Level 8 gym of Dragonites and Snorlaxes, with 2 Wigglytuff still alive (one 1/4 HP, one full HP).

Mons with High HP and/or elite move set are all good choices.
It's pity that Wigglytuff got nerfed and that she can't learn Body Slam. If she could, she would even better than she already is.

I do this frequently when training a friendly gym, don't recall ever attacking an enemy gym this way.

Offhand, I can remember doing this with a squad of six -
Butterfree
Persian
Pidgeot
Poliwrath
Tauros
Jynx
Jolteon
Raticate
Seadra
Victreebel
Parasect
Golduck

And I would totally do this with a 6-squad of Porygon, but unfortunately I only have two Porygon.

There are a lot of gyms around here (friend and foe) where the 6-Jolt Volt is a decent strategy. You must be at least average at dodging, though. Jolteon is a fun Mon to use.