He's been mentioned. No need. I didn't find him overly difficult though, maybe my teams just bawz! ¬_¬
Almost every pokemon has a downfall:
Rain:
Politoed's hypnosis misses too much generally giving a free turn.
Lanturn has stockpile. Fail. It might raise defenses, but it doesn't hurt that much. I have 1 OHKO moves on my team which makes it redundant, and spamming it just lets you whittle down the health which he has done in the past.
Tentacruel can't hit a multitude of types. Dragon (but lot's don't.). Ice (somehow). And it seems very interested on keeping it's defenses high with barrier, occasionally letting burn takes it toll and if the hax is good then paralyze as well. Besides, hydro pump being the only strong move any type that resists water is pretty much fine, since giga drain really doesn't hit hard at all.
Omastar doesn't really have one.
Kabutops has low kick. Low kick It's wrong on so many levels I'm not going to explain it to you.
Starmie is great, but can be taken down fairly easily.
Sun:
Ninetails has confuse ray and foul play. Confusion can be annoying. Oh god. Foul play... well.. Don't use a pokemon with a high attack! :P Also life orb is rubbish on it. Foul play doesn't get boosted by the item (I think, I don't use it in play but I'm pretty sure it's right) and for such a fragile pokemon without a recovery it truly is terrible.
Arcanine is great, however if you play well you can come in and hurt it while it is locked into outrage. Or get lucky with confusion hax.
Rapidash has a random special move. It has good coverage, I suppose, but drill run is too weak being neutral, flare blitz recoil can be fatal for a fragile pokemon like rapidash and the only good move is megahorn. Megahorn is great on a number of levels, the rest are not.
Shiftry... Man. Did this guy give some amount of trouble. Focus miss misses a lot though, but that's pretty much it.
Ninjask is random, has solar beam on a low special attack, 1 physical attacking move, which is X-scissor, and has protect, which is for speed boost. However, the point of protect speed boost is to use it for a free speed boost when he is sent out, but sometimes he uses it 2 or 3 turns into the battle and while annoying does nothing for it. It can not touch flying types, or lots of pokemon that resist bug (which is a lot) since solar beam won't hit hard enough even on rocks to KO it anyway (except rock/ground and some rocks like onyx).
Leafeon has attract. Damn. ANOTHER X-scissor puts 3 bug moves on his team for no reason, and particularly for leafeon it could be used for something far better. iron tail is generally bad, but at least he has leaf blade.
Best pokemon:
Starmie, Shiftry, Omastar.
Worst pokemon:
Ninjask, Leafeon and Rapidash.
Clearly sun is far worse than rain.. FAR worse.