Castle is the easiest, in my opinion. It's pretty much the same as Battle Tower, except you know what's coming up next.
Good job on the Gold. I've yet to get a Gold in any facility, though I've gotten Silver in Tower, Castle, and Factory. I'll try Tower again soon. I've made it to the 7th set of battles 3-4 times, but I always lose somewhere around battle 45. I don't really have any good Battle Frontier Pokemon, I just kind of use Pokemon I like with sets that are good in 1v1 6 Pokemon battles.
Most recently used Tower team:
Gengar @ Focus Sash
Shadow Ball
Thunderbolt
Hidden Power Fire
Explosion
Lucario @ Life Orb
Close Combat
Extremespeed
Swords Dance
Crunch
Milotic @ Leftovers
Hypnosis
Surf
Recover
Ice Beam
Another team that got me just as far:
Jolteon @ Leftovers
Thunderbolt
Baton Pass
Substitute
Shadow Ball
Breloom @ Toxic Orb
Spore
Substitute
Seed Bomb
Focus Punch
Milotic @ Leftovers
Hypnosis
Surf
Recover
Ice Beam
Good job on the Gold. I've yet to get a Gold in any facility, though I've gotten Silver in Tower, Castle, and Factory. I'll try Tower again soon. I've made it to the 7th set of battles 3-4 times, but I always lose somewhere around battle 45. I don't really have any good Battle Frontier Pokemon, I just kind of use Pokemon I like with sets that are good in 1v1 6 Pokemon battles.
Most recently used Tower team:
Gengar @ Focus Sash
Shadow Ball
Thunderbolt
Hidden Power Fire
Explosion
Lucario @ Life Orb
Close Combat
Extremespeed
Swords Dance
Crunch
Milotic @ Leftovers
Hypnosis
Surf
Recover
Ice Beam
Another team that got me just as far:
Jolteon @ Leftovers
Thunderbolt
Baton Pass
Substitute
Shadow Ball
Breloom @ Toxic Orb
Spore
Substitute
Seed Bomb
Focus Punch
Milotic @ Leftovers
Hypnosis
Surf
Recover
Ice Beam