At first glance, Swanna looks mediocre at best, with no base stats over 100. However, believe it or not, she's definitely a threat to watch out for. In rain, Swanna becomes a powerhouse with Hurricane, a move that sets her apart from other rain sweepers. Swanna has amazing offensive stats and a great Speed of 98, which allows her to outspeed many threats in NU, such as Gardevoir and Braviary. She also has access to Roost as well as Rest, which pairs well with her Hydration ability.
Unfortunately, Swanna is far from invincible, and she faces many obstacles in NU. Stealth Rock happens to be one of them, as Swanna has a weakness to it. Swanna's movepool is decent but not amazing, leaving her with only one good set. Despite boasting one of the best typings in NU, Swanna's subpar bulk holds her back; 75 / 63 / 63 defenses are far from great. However, Swanna is still an amazing ally to any rain team.
This is Swanna's best set for the NU environment. Boasting two STAB moves that benefit greatly from rain and a great Speed stat, Swanna has immense potential as a sweeper. Surf is the primary STAB move, getting a power increase from rain. Hurricane is Swanna's secondary STAB move, and it boasts a high 120 Base Power, perfect accuracy in rain, and a 30% chance of confusing the foe; this is something that even the most defensive Pokemon in NU struggle to handle if they don't resist it.
For the final two moveslots, you should use the combination of either Rain Dance and Rest or Substitute and Roost. With the former, Swanna can heal herself to full HP at any time as long as the rain she provides is up thanks to Hydration, making her much more self-sufficient. Alternatively, the latter lets Swanna heal without rain up and ward off status with Substitute. Substitute also lets Swanna take advantage of obvious switches and dodge Sucker Punch. The downside of this combination is that once rain subsides Swanna doesn't have a way to summon it again, preventing her from healing herself if she is inflicted with a status and from boosting the power of her Surf and accuracy of her Hurricane.Team Options & Additional Comments >>>
Swanna doesn't have a very large movepool, so she doesn't have many other viable sets. Damp Rock is an option, but it wastes Swanna's offensive power, which should be taken advantage of whenever she is used due to her sad defenses. Brave Bird is plausible, but it doesn't have much use. Scald could be used in place of Surf for the burn chance, but the power drop is noticeable. Tailwind and FeatherDance could be used for support moves. Choice Scarf Swanna is one of the few viable options left for her and is the only option if she isn't used in rain.
Checks and Counters
Swanna has her fair share of checks. Choice Scarf Rotom-S is one of them, checking Swanna with Volt Switch. It can't switch in, however, as it is KOed by Surf in rain. The same applies for any Electric-type, specifically Ampharos and Electabuzz, that resist Hurricane and can OHKO Swanna. Bulky Pokemon, such as Musharna and Lickilicky, can take several STAB Hurricanes and proceed to do as they please. Altaria is a counter to any Swanna that doesn't carry Ice Beam, as it can wall her all day without it. Revenge killers can outspeed Swanna and easily KO her; an example would be Choice Scarf Sawk, which can get rid of Swanna with Stone Edge. Articuno, Regice, and Lapras can take any of Swanna's attacks due to their high Special Defense. The latter two have access to Thunderbolt, which OHKOes Swanna.