Given this is a core I posted and then built around for my sample team, I figure I'll re-post one of the partners I used:
Roselia @ Eviolite
Ability: Natural Cure
EVs: 212 HP / 252 SpA / 44 Spe or 160 HP / 252 SpA / 96 Spe
Modest Nature
IVs: 0 Atk
- Sludge Bomb
- Leaf Storm
- Toxic Spikes / Spikes
- Synthesis
I gave two separate EV spreads: 44 Spe lets you outspeed uninvested neutral nature base 70s (i.e. Solrock and Politoed are nice benchmarks to outspeed for this set). The second spread lets you hit Golem super effectively and outspeed other neutral nature 252 spe base 45s, although that is not mandatory.
Roselia makes a nice partner to this core for a number of reasons: firstly, it absorbs any Toxic Spikes for Vigoroth, letting Vigoroth stay in rather than being forced out eventually due to Poison damage. Conversely, laying Toxic Spikes for Vigoroth can let Vigoroth break through many physical walls that it would have normally been hard pressed to do without Toxic Spikes. In return, Vigoroth can protect Roselia's Toxic Spikes (or normal Spikes) by Taunting enemy hazard removal out of using Defog.
Roselia also resists heavy Fighting-type hits such as Focus Blast from various special threats that can easily 2HKO Vigoroth even through its great Special Defense stat. And with Natural Cure, Roselia helps out Vigoroth yet again by absorbing annoying things like Misdreavus' Will-o-Wisp and other status moves that would otherwise disable Vigoroth.
Finally, Roselia can OHKO standard Piloswine with its 252+ STAB Leaf Storm, which is great for protecting Jumpluff from Piloswine during the course of a match (it is still recommended to pack a reliable switch-in for 'Swine though):
252+ SpA Roselia Leaf Storm vs. 248 HP / 8 SpD Eviolite Piloswine: 390-458 (96.7 - 113.6%) -- 75% chance to OHKO