Alright I'm not going to go too in depth on this so bear with me
First of all
jibblies out of my walls sometimes..
-_______-
Anyways let's settle down on the movesets shall we? Let's assume, for this part, that I change no member of your team
- Try to snag a Timid Darkrai if you can.
- I suggest going with the Boosting tank set on Kyogre since your team lacks any form of offensive prowess really.
- Blissey: Seismic Toss/Toxic/Stealth Rock/Softboiled - Your team needs a Stealth Rocker. That or Ho-oh will badly punish your team, because not even Kyogre will take on its absurdly powerful Brave Birds.
- Mewtwo: Taunt/CM/Ice Beam/Aura Sphere. You can also try Recover > Aura Sphere if you don't mind Darkrai all that much. Taunt/Calm Mind/Recover/Ice Beam can harass opsing stall a lot, and even more with Toxic Spikes support. Just be aware that you're off with one attack here.
- Forretress: Change nature to Careful, use this moveset: Payback/Toxic Spikes/Spikes/Rapid Spin.
DO NOT USE A -SPEED NATURE unless you want to lose badly to Wobbuffet. EVs: 252 HP / 6 Atk / 252 SDef
Something that's bugging me a lot with this team is
the lack of spinblocker. A good stall team will completely wreck your team by spinning away your hazards while putting theirs up (which you can't effectively spin due to them having a Ghost, which every stall team MUST have). Giratina-O would be a good option somewhere on your team, due to its ability to destroy Forretress and blocking Rapid Spin, while being immune to Toxic Spikes and Spikes through Levitate. It would have been a nice fit over Darkrai, who as far as I'm concerned doesn't do anything for your overall team. Unfortunately, if you're doing this on the DS Giratina-O isn't an option unless you 1) use an AR to allow it to be used on WiFi, 2) battle someone wirelessly, or 3) battle on Shoddy.
So with that said, your best option is possibly
Calm Mind/Dragon Pulse/Rest/Sleep Talk Giratina-A.
Next up I recommend Scarf Palkia > Dialga. Main reason being nothing in your team is able to take on a Choice Specs Water Spout from Kyogre. If you're concerned about Rayquaza, you can stuff some Defense EVs on Mewtwo so that it survives +2 Life Orb ExtremeSpeed from Adamant Rayquaza after Stealth Rock damage. 68 Defense EVs is enough for Mewtwo to survive after Stealth Rock damage + one turn of Leftovers from Adamant Ray (which kinda sucks tbh because Adamant Ray gets outsped by +speed tina-o and +speed Dialga, but that's for another story...). Alternatively, max HP/max Speed on Mewtwo can work, especially if you're going with Taunt/CM/IB/Recover.
You -may- want to consider Groudon over Kyogre here, if because Bulk Up Dialga and Kingdra are really annoying to your team. Sunlight also weakens Aqua Tails from opposing Palkia, and Groudon can provide your team with an extra Rayquaza check, as well as being able to phaze and set up Stealth Rock (which frees blissey up a moveslot). Since your team overall lacks the ability to take on physical hits, Groudon may be right for you. Groudon can also lead your team.
tl; dr: Your final team after these changes is Groudon/Forretress/Giratina/Blissey/Palkia/Mewtwo. You may choose to lead with Forretress instead of Groudon if that is to your liking
With these movesets:
Blissey - SToss/Ice Beam/Toxic/Softboiled
Mewtwo - Taunt/Calm Mind/Ice Beam/[Recover/Aura Sphere]
Palkia @ Choice Scarf (Hasty) - Outrage/Spacial Rend/[Fire Blast/Flamethrower]/Thunder
Giratina - Calm Mind/Dragon Pulse/Rest/Sleep Talk
Forretress - Spikes/Toxic Spikes/Payback/Rapid Spin
Groudon - SR/EQ/Stone Edge (for Ho-oh, mainly)/Roar
The lack of Levitating Pokemon may be bothersome, although CB Groudon is rarely used so you're not in a tight spot for the most part.
That is all. If you can use Giratina-O > Giratina, by all means do so as it provides your team with a better Spin blocker and Levitate. The optimal moveset to use for Giratina-O is HP Fire/Draco Meteor/Shadow Sneak/Outrage