Wacan Berry may have some uses against the likes of LO Latias or scarf-Rotom with the former which makes probably the best first switch into any Manaphy, and the latter that is an awesome revenge killer. To be honest though, I think that leftovers is still the best option on Manaphy, it helps compensating SS and SR damage, and considering that the two aforementioned Pokemon, along with Jolteon and the rare Raikou can be taken care by TTar, having an electric attack resist berry is not exactly necessary. And as the above poster said, opposing Manaphy will 2hko Manaphy regardless of the wacan berry, and it can't ohko in return so, once again, wacan is pretty much useless.