Specs Swellow is REALLY good right now, and I believe Piloswine is still one of its best partners. The main Pokemon that give Swellow trouble are Aurorus, Regirock, and other rock, ice and steel types. Piloswine helps swellow take these threats down with its impressive eviolite boosted bulk, and supereffective moves boosted by an adamant 100 attack. Piloswine also helps Swellow net some OHKO's it would have otherwise not recieved without stealth rock support.
For example:
252 SpA Choice Specs Swellow Boomburst vs. 248 HP / 0 SpD Eviolite Scyther: 153-181 (44.6 - 52.7%) -- 31.3% chance to OHKO after Stealth Rock
252 SpA Choice Specs Swellow Boomburst vs. 0 HP / 0 SpD Samurott: 255-301 (77 - 90.9%) -- 25% chance to OHKO after Stealth Rock
(There are many more OHKO's similar to this, but these are the 2 that sprung to mind first)
Furthermore, Piloswines ability to shield Swellow from electric type attacks further helps the longevity of this partnership. And finally, the icing on the cake of this core is that Piloswine can easily pick off foes Swellow has weakened with stab priority in Ice shard.