Reserving Amoonguss
Amoonguss @ Black Sludge
Ability: Regenerator
EVs: 248 HP / 168 Def / 92 SpD
Bold Nature
- Spore
- Giga Drain
- Clear Smog
- Hidden Power Fire
Calm Mind Manaphy appreciates having Amoonguss in order to switch into the grass type attacks it doesn't want to take. Because this particular variant of Manaphy is so slow, for instance, it will automatically be forced out by threats like Breloom, Ferrothorn, and Tangrowth. It doesn't appreciate dealing with mons that resist Water as well, as while Manaphy can click Calm Mind, other CM users like Slowbro, Reuniclus, and Keldeo use moves that target the base defense - which Manaphy cannot boost. CMers like Suicune, who do not have that option, can run Roar(and on Suicune, that's pretty much standard) to mitigate those boosts Manaphy worked so hard to accrue. Amoonguss can do the same to those CM users by clicking Clear Smog.
Also, one of the very nice things about Amoonguss is that it has access to Spore, which will essentially guarantee the need for your opponent to sack something as sleep fodder.
An important issue to deal with in teambuilding with this core is Latios, because it defeats Amoonguss relatively easily and can just fire off Dracos on Manaphy with ease.