Does Obito's Mangekyou Sharingan make him invincible? When Naruto, Kakashi and Guy had a battle with Obito. Obito was able to phase through Naruto's attacks, But Kakashi found a weakness, since his own and Obito's Sharingans were connected.

If Kakashi's Sharingan wasn't connected to Obito's. Would Obito have been invincible?
No, Obito has another weakness.

He cannot keep the "Everything goes through me" mode for more than 5 minutes straight. So if they kept going at him without letting him a chance to rest, he should have been dealt with.

However, this is a very hypothetical question, and thus can't be factually answered.