I've beaten the Gauntlet twice now, both times with radically different approaches. My first time, I used Drought Ninetales + Mega Houndoom, which is generally regarded as the best combo. The sheer power of Heat Waves coming off a base 140 SpAtk, sun boost, and Solar Power boost is insane. Some people also say use Charizard but in my opinion, Houndoom is the better choice. The power output from Houndoom is significantly higher - it OHKO's Radomus's Clefable outright with Heat Wave, which Charizard cannot do (this is helpful because of Clefable's Thunder Wave). It also does better against Luna's Gothitelle and Mismagius than Charizard so it has utility there too. Of course, you can also just get both pokemon and keep Zard in the back as backup, but if you only have the time and space for one, pick MegaDoom.

The second run I did, I used Trick Room. Slow mons with good spread moves can be very useful, such as Mega Ampharos, since it does a good job abusing the field. My team was Iron Ball Espeon (TR setter), Empoleon (Flash Cannon on the field is great + it had Mist and Brick Break to deal with screens / change terrain as I saw fit), Mega-Amphy (main hitter with Thunder/Dragon Pulse/Power Gem/Discharge), Sylveon (more spread move abuse with Pixilate Hyper Voice), Bisharp (sucker punch for Radomus), and Gastrodon (reliable tank that was immune to Discharge if I wanted electric spread damage). I liked the team I picked but it was definitely harder to pilot a trick room team since a 6-on-12 fight will last well over 5 turns, forcing you to reset the Trick Room. I def recommend sun spam, but if you have some slow mons you like, TR is also an option. No matter what though, have some spread moves.