I recently realized swampert isn't weak to any gyms, elite 4, and champion as mentioned above.
Fortree: Easy, as Swampert can get Ice Beam from abandoned ship
Mossdeep:Solrock and Lunatone are rock types, so: Easy
Sootopolis gym: Neutral, since water don't resist ground type moves
Sidney:Neutral, Ice beam for his cacturne
Drake:Easy , dealt with Ice beam
Steven: Steel types, lets see, oh:Easy
I mean all it takes is just a swampert with ice beam, earthquake, surf and mud shot.