Well when you're using Serperior, all you're really going to be doing is spamming Leaf Storm. So the main thing you want to watch out for is Pokemon who can eat up your STAB move. This includes Pokemon with Sap Sipper ( Miltank ), or bulky special walls who resist your STAB ( Amoonguss ).
Emboar
Well, this guy just DESTROYS. His STAB flare blitz and superpower completely wreck a lot of things that check Serpy. He can viably run Choice Band, Choice Scarf or my personal favourite, Expert Belt. What makes them so great as an offensive core is that Emboar kills off Amoonguss, Miltank, Sawsbuck & friends, while greatly appreciating Serpy effortlessly dispatching Alomomola and Rock types, letting him spam Flare Blitz like there's no tomorrow.
Gurdurr
Similar to Sawk, except he doesn't pack as much power. However, one of his extreme advantages is that he sets up all over Miltank and even Sawsbuck ( provided it's not SDed up, then you'll have to Mach Punch to victory ), bulking up and killing them with drain punch. He can even use Zebstrika as setup fodder to an extent, living 2 overheats thanks to the stat drops and KOing him, while recovering all his health :3 His one drawback is he's rather slow and often beaten by Amoonguss packing Spore or Clear Smog.
Magmar
I know you love this one :D with Vital Spirit, he is probably the best Amoonguss switchin in the meta barring maybe Swellow but that's 2HKOd by sludge bomb iirc. Eviolite is the way to go if you want to use this guy with serp. A simple moveset of Fire Blast / Cross Chop / ThunderPunch / filler would kill most things that can take a leaf storm.
Golem / Regirock / other generic rock type
Well these are great to back up Serpy, they can set up SR wearing down Flying & Bug switchins and turning 3HKOs into 2HKOs. But their main strong point is that they can safely switch into Swellow, who I'm pretty sure outspeeds you and kills you with its STAB brave bird. However, it can do nothing to these guys while they effortlessly crush it with rock blast or stone edge. Golem in particular can switch into all of the moves on Zebstrika's set except HP Grass.
Gardevoir
Not a lot to write about her, except her SubCM set destroys all Amoonguss and other poison types, she does compound bug weaknesses though so make sure to pack an emboar :D
Braviary
This is a short one too, but braviary is honestly one of the best Pokemon in this meta. It can run Choice Scarf, Choice Band and substitute + bulk up sets all perfectly viably, and can beat a lot of Serp counters one on one. It also, like Emboar, loves dem Rock types being taken out of the picture.
Swellow
You know the set; if your opponent's rock types are gone this poke runs rampage.
This is all I could think of at the moment, if I come up with some more I'll post them.
