I started off with the first two, Azumarill and (Get this) Landorus. They have incredible synergy, and with Latias, it made it completely clear to me that with Healing Wish I could get manychances to set up thanks to these three's typing synergy, how they attack both sides of defence, they wallbreak, they were seriously the entire package into three Pokemon. Belly Drum Azumarill with Healing Wish is fantastic, thanks to Azumarill's decent bulk naturally, it usually survives getting more than two chances with Healing Wish. Landorus, as explained above was chosen for the fabulous type, and attacking synergy. Rock Polish is amazing. I swear it makes sweeping soooo easy, I was thinking about using Modest but I rather be able to have a chance to take on other Landorus' 1v1 without setup. Latias just put cream on the cake Defogging annoying Webs, Rocks, etc that hurt the teams synergy (Switching out).
Afterwords I just looked through the team and see what it's quite weak against. First thing is first - Mamoswine/Ice Types, and Sand Rush Excadrill can ruin this team. So I put on Offensive Air Balloon Heatran as a rough counter, as with a free switch-in it can get an easy kill off of them. The EVs were from the OU Analyses, they let you outspeed Adamant Mega Heracross, and 2HKO Specially Defensive Mega Zard X (Given you have Air Balloon intact). I put on Venusaur quickly after I realized how well they worked together from past Stall Teams, and I made it offensive to deal with the terrifying Azumarill who otherwised smashed this team alive.
The last slot was hard, given I already had a Wallbreaker, Hazard Remover, Offensive Hazard Setter, Two Sweepers, a FWG core (I just noticed this as I was typing) and multiple ways to deal with many different Pokemon. I finally decided on Keldeo, Scarf Keldeo. It was a Counter to Mega-Gyarados with HP [Electric] (Which had a sudden spark in popularity) and Secret Sword, and it revenge-killed, the one thing my team was missing from what I saw. I struggled between Garchomp, but dropped it as my team would be even weaker to Ice-Types. Thanks to
A Terrakion for spotting out things I missed.