Not trying to call you out...but here are a few notes.
Thundurus-T beats Swamp. You'll need to use Grass Knot...but that isn't really a knock.
Timid with no S.Att boost does 72.27 - 86.13% to a Max S.Def and Max HP Swamp. Timid + LO to the Max S.Def and Max HP Swamp does 94.05 - 111.88%(68.75% chance to OHKO). Modest with no S.Att boost does 79.2 - 94.05% damage to that specific Swamp. Also an Expert Belt + Timid Set is seen with Grass Knot...it does 86.63 - 103.46%(18.75% chance to OHKO). HOWEVER vs a standard Swampert, Timid no S.Att boosted Grass Knot does 103.74 - 122.69%...so a standard Swamp isn't a problem in the least bit. Obviously Speed isn't an issue.
If you only have Black and White, not Black 2 and White 2 you can't use Thundurus-T(and Thundurus because he is Uber), so lets look at that.
Galvantula can handle Swamp easily.
Timid with Choice Specs does 82.17 - 97.02% to a Max S.Def and Max HP. Swamperts are typically Physically Bulky...so this would be a very rare set...but still manageable. Vs a Standard Bulky Swampert, a Timid Choice Specs or Life Orb, Energy Ball is a 100% OHKO. Modest with no S.Att boost does 85.78 - 101.74% damage(a 6.25% chance to OHKO). Again Swamp has less base Speed so you get to attack first, as well.
If that's not your cup of tea...lets look at Eelectross.
Really depends on the sets...but Max S.Def and Max HP Swamp is 2HKO'd(Grass Knot or Giga Drain...pick your poison) you'll need 282 S.Att(if your Timid at least 144 S.Att EVs...Modest needs at least 44 S.Att. EVs...also not Timid needs 32 Speed EVs to outspeed the Max S.Def and Max HP Swamp, Modest needs 84 Speed EVs to outspeed).
As for what Swamp can do to Eelektross, Choice Band Max Att has a 71.09% chance to 2HKO. Obviously it takes away from Swamp's ability to take hits.
Pikachu with Light Ball and a Timid Nature with Grass Knot OHKO's any standard Swampert. If the Swamp is Max HP and Max S.Att then it's a 2HKO(74.25 - 88.11%). Raichu with Grass Knot is a 2HKO at worst. LO + Timid has a 75% chance to OHKO the standard Bulky Swamp. Also Rotom - Cut(the Grass one) as you said can handle it.
Enough options for Electrics IMO. Just saying.