It must not be weak to:
It must be: - Fast - A decent special attacker (Good SAtk, decent special movepool).
I know nothing like this may exist but if it doesn't give the the closest thing possible :P
Like you already said the Lati twins is a good one. Seen on 8/10 Doubles teams. Also something that resist a BoltBeam combo is kinda hard :P.
[email protected] only has problems with Ice. Also Volcarona pops up in my mind, but she has a 4x Weakness against Rock but with the right support she can't be tamed. Water is easily covered by Giga Drain or any other Grass attack.