My first try at a decent team got me a streak of 74 wins.
First, the team:
Darmanitan @ Choice Scarf
EVs: 252 attack, ~ 200 speed, the rest in other (Was in the party for a bit before I started EV training)
lvl 50 stats: 185/210/77/51/75/120
Suicune @ Leftovers
EVs: 252/0/238/0/0/20 (I guess I somehow put some EVs back in Pearl)
lvl 50 stats: 207/86/172/106/135/97
Garchomp @ Yache Berry
lvl 50 stats: 183/199/102/85/105/153
The strategy was fairly straightforward. Darmanitan would Flare Blitz if it hit neutrally, Flare Blitz if it was super effective, and Flare Blitz in most cases if it was resisted. Darmanitan is almost always good for a KO or two, and despite the Brave nature, 180 speed outpaces most of the Subway. Most of the problem Pokemon that outspeed it are electrics and easily handled by Garchomp. If there's a bulky water, I switch in Suicune, who sets up to +6/+6 and sweeps. Suicune is also commonly the target of U-turns against sturdy PKMN. Even with 4 def IV, it's pretty hard to kill. Garchomp is there to keep up the offensive pressure if Darmanitan is revenge killed. Yache allows Chomp to survive almost all ice attacks, sneak in a Swords Dance, and sweep. I started out using Toxic on Suicune, but switched to Ice Beam after I realized that my team had a distinct Jolly dragon weakness.
Proplem Pokes: Offensive Starmie (Switch to Suicune, pivot to Garchomp on the Thunderbolt, eat Ice Beam, KO with Outrage), Latia@s, Garchomp (Flare Blitz, eat Earthquake, send in Suicune for the 2HKO), Salamence (U-turn out of intimidate, pray Suicune can get the burn, and if it doesn't bring back Darmanitan to Rock Slide)
The loss: Gentleman Camus (Latias, Suicune, Landorus)
Turn 1: Darminatan U-Turns out into Suicune, who eats a Specs-boosted Draco Meteor to 51 HP (Latias in yellow)
Turn 2: Latias KOs Suicune with Draco Meteor. Switch in Darmanitan
Turn 3: Darmanitan KOs Latias with Flare Blitz (praying he doesn't have a bulky water). He sends out Suicune (!@#$)
Turn 4: Flare Blitz crits, triggers Sitrus Berry. Darmanitan gets KO'd by Hydro Pump. Send in Garchomp.
Turn 5: Garchomp Swords Dances, eats a Blizzard to 73 HP.
Turn 6: Garchomp KOs Suicune with Earthquake. (Praying his last guy is slow). He sends out Landorus (okay...)
Turn 7: Landorus KOs Garchomp with Earth Power.
If I would've kept Darmanitan in and taken the KO, Suicune could've set up against a weakened Draco Meteor, let Latias die to struggles, get to +6/+6 and beat Suicune, then 1HKO Landorus with Ice Beam.