So... who's idea was it that Highlander shouldn't get a Heavy Attack after he Parries a Light Attack? Because I cannot count the number of times that I've Parried a Nobushi's Light Attack, as Highlander, and all I can get off is a Light.
Use Balor's Might. Leave your heavy attack button pressed for a second while you parry and Highlander will immediately follow the parry animation with his unblockable.
It's weird but you gotta get used to holding heavy once you hit heavy to parry. It lets you fast flow into offensive form where you can get a Ballors might for 40 damage.
Only time I think highlander gets a top heavy off is after a splat.