Hi there Princess Bri, excellent team and sorry I wasn't able to rate it earlier.
Anyways I can see CM Reuniclus as a pretty big threat, if it's the last Pokemon on your opponents team and it's up against either Hippowdon, Skarmory, or Heatran. In addition to that, it can also simply tear apart your team slowly, as it can constinently come in on Heatran and hurt it with Focus Blast, which could possibly open up a sweep for another Pokemon. The same is the case with Skarmory, as Reuniclus will often force you in the corner of Recovering or Phazing it after it damages one of your walls, and while the latter choice is usually the better one, this can easily open up sweeps for other Pokemon; Hippowdon is outright set up on. If Reuniclus is the last Pokemon and it comes out against either Heatran, Hippowdon or Skarmory, then it will sweep almost every time, given that Excadrill is OHKO'd at +1 and no combination of your moves can 2HKO it at +1, since Jellicent's Shadow Ball is only doing 27.59% - 32.55% to +1 Reuniclus, while Reuniclus is doing 40.35% - 47.77% with Psychic, so you'll eventually have to Recover soon after, plus you run into the paradox of attacking or taunting. I can also see CM Manaphy being somewhat problematic if you happen to use the weather war, as it will be outspeeding Excadrill, so you can't KO it with Earthquake, and it will also be able to shrug off Amoognuss's Spore, which is your main means of beating it outside of Rain. Additionally, while Jellicent is answer to most rain abusers and water types, it can't really touch CM Manaphy at all and even if it had a status move, it still couldn't thanks to Hydration. Anyways, I think you can get of these problems fairly easily by using a Defensive Perish Song Celebi over your Amoongus. Defensive Perish Song Celebi gives you an additional check to rain and also stops the aforementioned threats cold, thanks to Perish Song and the natural resistances that its typing gives it. EVs are directly from Ojama's RMT, as I'm sure you're probably aware, so I'm not sure exactly what they do, so feel free to play around with them.
Celebi @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 64 Def / 148 SDef / 44 Spd
Calm Nature (+SDef, -Atk)
- Perish Song
- U-turn / HP Fire
- Giga Drain
Aside from that, there isn't much else; this is a really well-built team and I'm glad to see stall can do well in DW OU :'). You could potentially put Stealth Rock on Heatran over Protect to free up a spot on Hippowdon for Roar, since in my experience, Tran generally has an easier time getting rocks up and Roar can prove really helpful in phazing stuff like Conkeldurr or Scizor out if the need ever arises. GL~