I’m surprised I haven’t seen more posts about this fight.  It shouldn’t be that hard to find, since sans flat out tells you where to go.  If you head back to Grillby’s after beating Sans you’ll find him waiting there for you.  And he’s not happy.

The music cuts in here.  Obviously there isn’t a full hidden track–instead, I’m pretty sure what plays is just MEGALOVANIA reversed (NEGALOVANIA?)  You can listen to it here: https://soundcloud.com/scarodactyl/negalovania-grillby-bonus-boss-theme

The glasses scroll from right to left or left to right, and they get faster each time.

This one gets really tricky, since the bubbles don’t move in straight lines and you can’t stop moving.  It’s not impossible but I’ve never avoided taking damage.

And sometimes he just lets one of these loose.  This one doesn’t look too hard to avoid, and it isn’t at first.  But it can strike in the center or on either side, and he’ll sometimes do it in the middle of the other two attacks.

As you would guess from his defense, one hit is enough to take him down…but he immediately reignites no matter how many times you do.

The only way to make it stick is to make a detour to Hotland first.

If you carry the cup with you into the battle you have a new option in the act menu.