Ah, a basic boosting Sun team. Well, the easy way to beat that is simply to add a DrizzleToad or a Tyranitar to your team. No kidding. Take away the weather and suddenly the team is no longer as dangerous. I would also go for Stealth Rock to make Ninetales think twice about switching in, which a TTar can also do.
It seems to me the two biggest threats on his team are Ninetales and Cloyster, as the others all rely on these two for support and thus are useless without them. As such, a decent fighting type would be very helpful, especially if it has access to rock type moves as well (Stone Edge anyone?). I'd argue LO Mienshao could knock a good chunk of that team out with a set of Stone Edge/U-Turn/HJK/HP Ice, running a Naive/Hasty nature. The good ting about Mienshao is that your opponent has no Ghost pokemon or Protect users, meaning aside from a WoW from Heatran, there is nothing to stop Mienshao from just spamming HJK all day long. Regenerator means LO won't take a huge toll, and HJK means Cloyster with think a lot before coming in, while Stone Edge keeps that blasted Ninetales at bay, and HP Ice scores SE damage against Lilligant. You could opt to run Fake Out in place of Stone Edge, but you lose out on the SE damage against Ninetales. Oh, and watch out for Victini, as its Psychic Typing resists HJK, so make sure to hit it with a Stone Edge on the way in. That thing (like all pixies) has deceptively good bulk.
So...yeah...Politoed + LO Mienshao would be my choice, maybe with a few more pokemon who can take advantage of rain (Hurricane Dragonite anyone?). Alternatively, run Kingdra with Politoed and Mienshao (note: use Sniper not Swift Swim on Kingdra due to Aldaron's Proposal) and abuse that 4x resist to fire and neutrality to grass, while hitting hard with Outrage/Draco Meteor/Waterfall/whatever you feel like. Hell, ChestoRest Kingdra could rip through half that team (assuming anyone still uses ChestoRest anything anymore these days).