Playthroughed with Gulpin again a couple of times these past few days. Swalot's main appeal is that it has a decent amount of disruptive moves in its repertoire, but Swalot's middling stats mean that it is somewhat moveset-dependent against certain matchups at a time. This does mean Swalot is rather open to customisation and results can vary greatly across various runs depending on what you give it, so I'll just go over its more notable moves and you can assess their importance for yourselves.
Swalot has a bunch of rare & useful utility options that you absolutely
need at least one of to make the most out of Swalot, which include:
Yawn: This is generally Swalot's most useful support move as stated by
shinquickman; the delayed but guaranteed sleep is basically an open invitation for your wincon (which can include Swalot itself) to get their headstart of piling on the X items and sweep for game. This will also primarily be Swalot's main contribution for fights where it otherwise performs poorly in, such as Wattson, Phoebe, and Steven.
Encore: This is a good alternative to Yawn in enabling X item abuse sweeps, and Swalot could even make use of Encore to disrupt certain setup-happy opponents such as Flannery's Slugma & Torkoal, Wallace/Juan's Luvdisc, Glacia's Sealeo, Drake's Shelgon, and even Steven's Skarmory even harder than Yawn can, which can open up greater opportunities for Swalot to try and X item sweep.
Toxic: The nice thing about Toxic on Swalot is that it learns it naturally at level 31, which is not far from the time you get access to the Toxic TM in Fiery Path so you don't even have to go out of your way to get it. This can be used to try and bring down annoying fatmons like Phoebe's Dusclops, Glacia's Walrein, Wallace's Ludicolo (
bonus points if you have Liquid Ooze) & Milotic, and Steven's fossils, though Toxic stalling is generally paired best with...
Amnesia: Amnesia can already prove useful even before Swalot gets Toxic, as it can be used to let Swalot tank through Flannery, but other than that, it's best used alongside the aforementioned Toxic to stall out bulky Waters.
In terms of offense, Swalot will be content with Sludge + Body Slam for the pre-Surf portion of the game (again, one of Swalot's main perks is that it has fairly solid attacking moves compared to the majority of pre-Surf Pokemon), then afterward Sludge Bomb and/or Ice Beam will take over from there. It is also notable that with Yawn/Encore facilitating X item abuse, these moves can even have sweeping potential despite Swalot's middling offensive prowess.