Tenten,in the right hands,is possibly the strongest character in the game.

If you play as her,it's best to keep your distance and keep throwing shuriken. Tenten's normal combos are slow,but her shuriken combo are really fast (and annoying if you're fighting against her. almost as strong as deidara's)
Her ninjutsu's pretty good,it works well in mid range.Her tilt's pretty average...Her Ultimate......well honestly I never used her ultimate online. I think it's mid range though.

The key if you're playing her is to keep your distance.
The most powerful weapon in Tenten's arsenal is her instant awakening. Shuriken are turned into fireballs,chakra shuriken launch lightning down on the enemy and work brilliantly if he's charging chakra, Her fireballs overpower most ninjutsu (still no match to amaterasu though). Tilting lets you use an earth style attack,great in close range. Awakening actions let you use the Water Style (great at chasing the enemy) and wind style (pretty fast).
That's pretty much it that Tenten can do. If you're in close range,it will be hard to use your shuriken. Try to use the down combo instead.
Despite all I said,Tenten is not OP. Unlike Deidara,you don't need to be Minato,Guy or Konan to dodge her attacks. Only the last weapon in her combos can knock you down. The strategy is to keep moving. You must stay close to her or at least be able to hit her. I'd say having Uchihas in your team is a very good idea. Tobi is also a very good idea to have,since he attacks her from underground.
Beating Tenten is not a huge challenge. If you know how to be an expert with her,you also know how to beat her.
It's a BAD idea to sub against any spammer's attacks. Try not to do it. Your guard won't last 2 attacks in her awakening. Her lighting attack is a perfect guard breaker.
If you wanna beat Tenten- keep moving. Her defense is weak. To get close to her,just chakra dash. Her shuriken won't push you back (but the fireballs in the awakening will)
There's a lot of characters that can beat her. Just don't try Utakata though (i still don't understand how to use him :P )
Pretty much everyone,from Nagato to Kakashi has a good chance against her. If you know all about how Tenten works,it's almost impossible to lose to her. And hard for you to lose when you're playing her.Minato would be a problem to beat though.Characters good in close-range combat are excellent against her.She's perfect when it comes to facing other spammers (especially Deidara)
Hope it helps :P