Oh, this is what I get for not reading. The bubble is in the room just after first boss, on the left behind the tomb. Mage invisibility lasts 20 seconds so if you are exactly right you can get past the boss in one hit, then take a lesser invis pot (15 seconds) and run around hugging the left wall into the back left corner.
Not sure where people relying on potions are going to get past the boss.
Not sure where people relying on potions are going to get past the boss.