Manaphy is an extremely bad choice for NDAG, as a whole.
As an offensive Water-type, Manaphy is outclassed endlessly by Kyogre, Kyogre-Primal and Arceus-Water. Even though that strategy might seem appealing, it ain't good in NatDex AG. Here's why:
Mediocre stats:
Manaphy has 100 base stats across the board. Now, this might seem pretty nice, but it isn’t at all. With only 100 SpA, even after a Tail Glow boost, it wouldn’t be doing anything to bulky walls such as Arceus-Water. Its defences are also awful. 100/100/100 is nothing to boast about. It’d get stepped on by almost all of the most common offensive Pokémon in the current metagame. Overall, its base stats remain bad for NDAG.
Relies on a lot of turns to deal even a little damage:
This is an even bigger issue for Manaphy. It’ll rely on more than one turn to set up, which’ll give the opponent time to play in a way which would inversely affect you. Its overreliance on a Z-Move, which doesn't do anything notable, makes its condition even worse. It’ll get revenge-killed soon enough with that pitiful bulk anyways, and these turns would drench it a lot.
Outclassed as a Water-type:
As an offensive Water-type in NDAG, look no further than Kyogre, Kyogre-Primal and Arceus-Water. All three of them have their perks, and you can choose one from your team. Normal Kyogre functions primarily as a Choice Scarf or Calm Mind Dynamax sweeper and is an extremely threatening offensive Pokémon. It can invalidate its checks and also sits on Arceus-Ground, one of the most common Pokémon in NatDex AG. Kyogre-Primal also functions as a wallbreaker/late-game sweeper, but it doesn’t have the luxury of an item but has better stats. Arceus-Water is another beast that can set up on passive Pokémon like Necrozma-Dusk Mane and is difficult to check after acquiring a few Calm Mind boosts.
These all Pokémon are, unarguably, better than Manaphy in NDAG. You might just want to use any of these, according to what fits your team.
