Palkia and Latias both fill similar roles and thus when teambuilding you should pick just one. Palkia has more offensive presence while Latias has Levitate and an immunity to spikes. Latias + Specs Ogre is a nice combo, but Kyogre isn't going to hover above those spikes Ferro can spam against Latias.
This brings me to my next advise on rain offence: hazard control. As I mentioned earlier, rain teams can have trouble with Ferrothorn and this means spikes on your side of the field. Of all the Pokemon we have discussed in this thread, only the Lati twins and Rayquaza don't mind spikes, and Rayquaza obviously is weak to rocks. So you need to have a way to spin those rocks away and keep your Specs Ogre at full HP or for your Palkia to have a chance against enemy Kyorgre. Lets have a look at Uber spinners (format will be HP / Def / SpDef):
...
Forretress
75 / 140 / 60
- Spikes
- Stealth Rocks
- Rapid Spin
- Toxic Spikes
- Volt Switch
Forry is quite the team supporter, boasting all hazards, rapid spin, and volt switch for momentum against against spin-blockers. Oh, and he's a Mewtwo check in rain !
The problem with Forry lies in his competition with Ferrothorn for a team slot. Forry isn't a wall like Ferrothorn, he is a team supporter. If you try to use him like a wall he will just die prematurely and you will have no team support. Ironically, the best time to send Forretress out is against Ferrothorn, because Ferro literally can't do anything to Forry.
...
Tentacruel
80 / 65 / 120
- Rapid Spin
- Toxic Spikes
- Rain dish
- Absorbs Toxic Spikes
Tentacruel is also more than just a spinner. He brings Toxic Spikes to the table and takes them away just by switching in. There are downsides to using him on an offensive rain team, however. He adds another electric weakness as you already have Kyogre; He can get set up on by many physical threats if you dont get a scald burn and pretty much any CM Arceus that comes in before toxic spikes (or on 1 layer); and he just isn't that bulky. Just as Forretress you can't use him like a dedicated wall, which can be a problem on an offensive team.
...
Excadrill
- Rapid Spin
- Stealth Rock
- Electric Immunity
- Poison Immunity
- Offensive Presence
- Mold Breaker
Ok, I'm just gonna say it. Excadrill is the best spinner for an offensive rain team (imo). That list above pretty much says it all. With mold breaker, you can hit Giritina-O with a STAB Earthquake, wearing it down easily as it has no recovery outside of rest (which is super rare). He can even beat dragon tail Giritina-A with sub + swords dance. He is also a Zekrom check which is amazing because Zekrom can run through a lot of rain team members. Try Excadrill out. It is much more than a sand sweeper !
Hope this was helpful !