press lc/m1 only once, then use three, one its a unblockable combo if you land the first part. The full one is lc/m1 once, three, one, two, lc/m1 once, four.
I honestly find mg21 to be pretty bad in this game and heisei being a direct counter almost, any attack mecha tries that includes a rush, a good heisei can easily counter it
Nice video man, i like kaiju arisen, and i know its pretty radom but Repent and believe the gospel, this could be your last chance to be saved, God bless you