The parry gives the opponent GRD meter. I don't think its a good idea to just use it against multiple hit moves like Merkava's EX "I, pierce". Its best used sparingly. Just take the one parry unless you want to show off. If you want to punish Merkava's reversal beat it with a upper cut from mid range if your positive they will use it or shield the last hit and punish with 2a if your closer.The shield option is unreliable but possible.