Grampa the Grey

[Trapfly]

I recently completed Wizards of the Square Tablet on hard mode, earning gold stars on each level. Despite this, I'm still horribly confused about the "Surpassing the Master" achievement, in which you're supposed to defeat Grampa the Grey.

I've seen the character in question throughout the game at certain points, but I have absolutely no idea how to trigger some sort of battle with him.

Now then, if this is being kept a secret deliberately, that's fine. If it isn't, however, I'd appreciate at least a hint about what to do.
Thank you for your time, and have a wonderful day.

 

[Trapfly]

Well, after a week, I finally found Grampa. Since the reward is quite nice, I suppose that keeping it very secret was a good call.