Phione's main niche in the UU metagame is to function as a bulky rain supporter. With its ability, Hydration, Phione is able to repeatedly switch in and set up Rain Dance while Resting off any damage taken. Hydration also allows Phione to act as an excellent status absorber for rain teams, preventing its fellow rain sweepers from being crippled. Waterfall is Phione's main STAB move and hits reasonably hard in the rain. U-turn is the icing on the cake, allowing Phione to scout the opponent's switch-in while enabling rain sweepers to come in unscathed.
The EV spread maximizes Phione's bulk, while allowing it to Speed tie with its fellow base 80s, most notably Venusaur. If you want to play Phione more aggressively, an EV spread of 4 HP / 252 Atk / 252 Spe can be used; however, Phione won't be able to set up Rain Dance nearly as many times as it can with the main spread if it chooses to utilize such an offensive EV spread. Damp Rock is the item of choice, allowing Phione to create an eight-turn Rain Dance. Heal Bell can be used in place of U-turn to cure status for your team, though U-turn is extremely useful for scouting purposes, one of the main reasons you should even consider using Phione on your team.
It should be noted that by using Phione, you are adding additional Electric- and Grass-type weaknesses for your Rain Dance team to struggle with. For this reason, Pokemon that resist these attacking types are essential teammates. Grass-, Dragon-, and Ground-types, such as Tangrowth, Altaria, and Rhyperior, can cover these weaknesses well, while forming decent defensive cores on a Rain team. Since Phione is dedicated to setting up rain for the rest of the its team, Pokemon that can abuse the rain are essential teammates. Kabutops, Ludicolo, Qwilfish, Omastar, and Gorebyss are all extremely deadly sweepers in the rain, making them excellent partners.
Toxicroak and Poliwrath are the biggest threats to Phione and its team. These Pokemon can switch into Phione as it uses Rain Dance, and set up Substitute, Nasty Plot, or Swords Dance as Phione switches out. Uxie and Weezing can wall Toxicroak and Poliwrath without much difficulty; additionally, both Pokemon are able to set up Rain Dance themselves, making them wonderful teammates. Venusaur is a very good teammate to Phione because of its ability to switch into Grass- and Electric-type attacks with ease. It can also absorb Toxic Spikes, which are quite troublesome for rain teams.