Swampert for Heatran Raids.

Assuming Mud Shot + Earthquake Swampert: Bug Bite + Iron Head it's 2nd best after Groudon. Fire Spin + Iron Head it beats out Rhydon. Any other move combination and Groudon, Rhyperior and Rhydon are better.

Swampert doesnt resist bug. It resists all of Heatran's charge moves, but only half of its total sets.

If you have one evolved, i'd say yes, but if you dont, keep on as a mudkip/Marshomp and wait for its CD, Rhyperior and Rhydon are a lot less risky to invest in (if it gets a CD moveset, theres still the other STAB type for it to be great at using, other than swampert which is just average as both water and ground without hydrocannon for the water side and ground being a below average type offensively in geeral)

Me and my raid partner duo Heatran with 4 Swamperts between us. Hasn't failed us yet. I prefer him to Rhydon because Mud Slap is excruciatingly slow

I have been extremely happy with my Swampert’s performance in the lead spot on my second squads in Heatran duos

It's one of its best counters, yes.