PSA: Shadow Palkia will also be an excellent Water-type attacker (with Party Power).

Using Dialgadex as an indicator, shadow Palkia is essentially a "sidegrade" as a Dragon-type counter. It ranks #4 overall, just behind Mega Rayquaza/Garchomp/Salamence, which makes it the best non-mega Dragon type (assuming perfect IVs). When accounting for Party Power, however - Origin Dialga and Palkia both outstrip it in terms of DPS (meaning it drops to #5 overall, with a higher candy/stardust cost).

This is probably pretty well-known as obviously, that's where Palkia will be most relevant. But Palkia is also a Water-type. Now, if you look at its ranking, it's not exceptional. It ranks #13, just below Primarina (which is far easier to farm candy for and again, less stardust). But throw in Party Power, and this changes entirely (rank #7 overall, and #3 when excluding Megas/Primal Kyogre). Only the 3 Kyogres (Primal, shadow, regular) really surpass it.

Why is this? For starters, Dragon Breath (while not a Water-type move) is really good at charging energy for charged moves. And with Party Power, shadow Palkia prefers Hydro Pump (a standard 1-bar nuke move which benefits greatly from PP) over Aqua Tail. With that neat combo of fast spammy move + powerful nuke boosted by PP, shadow Palkia is a pretty good Water attacker.

Entei, for example, ranks it as #6 best counter, only behind the Primals, their shadow variants and Mega Swampert when accounting for rainy weather (which can be replicated by having Primal Kyogre's boost). Palkia not needing Elite TMs to swap between moves also helps its versatility greatly.