Choice Specs Heatran was decently used in early DP ubers but Heatran is rarely seen (#30 in Decemeber, less use than Gengar) now because it has trouble switching in, particular in the hyper-offensive uber metagame. It "can" work but I just see it being too specialized to be used effectively. It basically just counters Latias that lack Safeguard / Refresh (and it needs Toxic to do it otherwise it'll just CM up and sweep). Even the standard Latias Thunder will 2HKO this Heatran 100% of the time with Stealth Rock so it's questionable even then. With Kyogre rampant Heatran is even in more difficult to use effectively since its Fire STAB is lost (and really - Groudon isn't exactly something that can switch in to things over and over again so your Sun is often limited).
Heatran actually has many chances to switch in, because many Choice attackers throw around mostly Dragon and Fire attacks, without a fighting/ground attack to back it up (see MixQuaza), so he often does get a lot of chances to switch in.
Sun is not as limited as it seems, as Groudon can switch into most support pokemon, and his immunity to Electric means that as Kyogre tries to Thunder, say, your Palkia, you can force it out, get Sun up, and force it to switch into Stealth Rocks again sometime, losing its water spout damage output, and allowing pokemon like Dialga to wall what remains.
But I guess you could say it is a bit specialized. I have Scizor on my team, and he and every other steel fears switching into a Specs Dialga or something, because of Fire Blast. I also happen to have Kyogre completely covered, so Heatran's ability to switch into a lot is very useful.
I have a feeling that this set might be
epic fail in terms of actual usefulness (I found it useful) but....
I needed a good mixed attacker in UU. Now, most, if not all of the best ones are Fire type, and Nidoking is sorta there too. Fire obviously sucks, with their SR weaknesses. All of them are fairly slow too. All three cannot really beat Milotic. They could cripple her, possibly, but she can use recover very easily. Milotic is easily one of the best walls in UU, as it can take all but the hardest hits on both ends of the spectrum, and basically ruins most mixed attackers chances.
Then I thought about Shaymin. I realized that besides pokemon that are faster, and scarfers, very little besides Chansey can actually stop the thing. Most other walls, like Spiritomb, Steelix, the bulky waters, etc. are all beaten by Shaymin anyway. So all you need to do is kill Chansey, and of course, Shaymin will have to use its only half decent physical attack to do so, Seed Bomb. Regice walls Shaymin too, but Regice has to take 25% on the switch in.
I've used it a couple times in quite a few matches, but I only faced three Chansey. I did kill them though.
Mixed Shaymin
Shaymin @ LO
Natural Cure
208 Atk/120 SpA/180 Spd
Hasty
-Seed Flare
-Seed Bomb
-Earth Power
-HP Fire/Air Slash
Besides pokemon that simply kill Shaymin by outspeeding it (like Staraptor), and the joke that is Regice, only Chansey can really stand in the way of a Shaymin rampage in nUU. Sure you can theoretically beat it with Seed Flares SpD drops, but it is unreliable at best, and Chansey can easily switch out to something else and get rid of the drop. So this Shaymin invests a bit into attack to eliminate Chansey with Seed Bomb.
With 208 Atk EVs, Life Orb, and Seed Bomb, Shaymin can Seed Bomb as Chansey switches in, and then once more for a 2HKO 96% of the time. Even if Chansey uses Protect to block the 2nd Seed Bomb, the odds of you killing her overall is greater than 75%, which is a pretty good margin for error really. However, much like MixMence's Brick Break in OU, you need to hit Chansey on the switch, otherwise she can paralyze and spam Wish or Softboiled. You can also bluff SD Shaymin, then pop something with a Seed Flare later on.
120 SpA EVs with Life Orb is just enough to get a 2HKO more than 90% of the time on RestTalk (252 Hp/160 SpD Calm) Spiritomb after SR, and will also 1HKO the standrad 148 HP/0 SpD Milotic 80% of the time, dealing 85% damage minimum.
Durability on this Shaymin is a bit low, as between SR and LO and switching into hits, Shaymin isn't going to live long. Though a healing move would take away a lot of coverage, you can put Rest or Synthesis over Air Slash/HP Fire/Earth Power. Leech Seed would be fairly pointless, since Shaymin should beat Chansey with Leech Seed by itself.
180 Spd with a Hasty Nature so that even with HP Fire (which reduces your Speed IV to 30), you can outpace base +natured base 90s, notably Roserade. Air Slash can be used over this, which means he get an extra 4 EVs to place somewhere else. Air Slash can hit fighters hard if you don't want to use Seed Bomb, but Fire/Ground/Grass is very good coverage. Additional speed EVs can be put in here to outspeed +speed natured 95s, notably Arcanine (though I am not sure how many max speed). Hasty is chosen because Shaymin tends to switch in more on the special side (T Bolts, Surfs, etc.)
Issues: This plays more like the standard LO Shaymin; it has just enough SpA to still kick ass and take names. The lack of recovery is a bit disappointing, but this usually nets a few kills. Seed Bomb is pretty effective at killing Chansey, and I know that I did need to crack one stall team.
Not sure whether I need HP Fire at all. Grass/Ground/Flying seems to still have good coverage, so I figure its not really necessary.
Still, I wonder whether SubSeed or just plain Leech Seed Shaymin pulls it all off better.
Regice "walls" this but it takes 25% from SR switching in, and takes 30% minimum from Seed Bomb. So eventually, you will win. And if it Rests, you can easily bring in something like Gallade and force it out again. In otherwords, Regice sucks.