I've had variants of this team kicking around in my teambuilder from forever ago, all centered around the core of Manaphy, Tangela, Rotom-Heat, and Mew. It's basically bunnies for DUU (Azumarill=Manaphy, Tangela=Amoonguss, Rotom-Heat=Heatran, and Mew is Mew), but I explain the core in more detail
here. The last two slots I wanted a mega and a Liepard check. I decided Aero because it helps Manaphy set up with Sky Drop support and Wide Guard blocks stupid ladder Discharge teams or other spread moves such as Rock Slide. I decided Virizion for my last mon kind of at a whim, since I really needed a fast Quick Guard user to shut down Liepard but at the same time I already had Tangela for a bulky Grass-type. Since no better option seemed to jump out at me, I just decided to go with it.
I used Tangela before it was cool