Guarding an attack puts you in the Guard stance, during which you invincible. The invincibility lasts long enough to mash Guard to immediately go back into the stance afterwards. Counterguard lets you press attack during the Guard stance for an AoE strike that breaks super armor and is invincible. Guard is great, though not used much against normal enemies for being constantly on the offense.

You are invincible during all Limits (but some of Limit Form's) so long as the attack is going off or the time gauge is decreasing. Limits tend to be very strong (though not as strong as the best normal strings) and have very high stun. Not all Limits are used the same way. For some, your best off mashing triangle (Knocksmash) others pressing Triangle intermediately to draw out the I-frames, some alternating between Triangle and X (Mulan's) and in the case of Jack Skellingtons, letting the Limit go off as it does damage regularly. Standout cases include Tron's (best Limit in the game, bar none), Goofy's (Knocksmash deals high damage while letting you attack normally during the last portion, Whirli-Goof is the strongest Limit available to Sora/Donald/Goofy), Comet (I-frames and AoE), and Trinity (hits all enemies when Sora/Donald/Goofy are all alive, acts as Sora's standalone).

Note most Limits deal most of their damage at the end, with the prior hits powering up the finisher (more hits=more damage).