Hey!
You looked hard-pressed to take rain-boosted attacks. Kingdra's LO limits its switch ints while Latios just doesn't have the bulk. Meanwhile, Keldeo can come in and threaten everything (bar a Latios that can't switch into Icy Wind / HP Ice). LO Breloom wrecks everything if it can get a free switch-in. Finally, Specs Latios and LO Kingdra seems like it can just run over everything.
I would firstly replace Forretres for
Ferrothorn. Ferrothorn is much bulkier, so it can take rain hits a hell of a lot better, while giving you an answer to opposing Kingdra and Specs Latios. The bummer is you can't spin rocks, but Gyara will usually only set up when its got the game anyway so you really don't need to spin. (Standard EVS with
SR / Gyro Ball / Power Whip / Spikes or Leech).
On Gyarados, I would replace Stone Edge for
Bounce, giving you both a better answer to Keldeo and an answer to Gyarados. Change his Evs to
8 Hp / 252 Atk / 248 Spe. I can't remember off the top of my head but those speed EVs are crucial to out speed certain threats after +1.
I would replace Stone Edge for
Icicle Spear on Mamoswine so you can hit those Sub Gliscors and those Multiscale Dragonite's for the KO, while replacing Icicle Crash for
Superpower to hit Rotom-W.
Breloom wants
66 Hp / 252 Atk / 184 Spe, and the Lum Berry is kind of redundant. Run
Fight Gem instead, as the extra boosted power allows you to revenge faster threats if you lack rain and Latios is down.
Try
Psyshock on Latios over Hidden Power Fire to hit Keldeo and Terrakion, while you should also keep in mind that Breloom is the best counter to Ferrothorn so you have no need for HP Fire.
Good Luck!
~Shurtugal~