How can I get into the locked door

Hint 1 of 8:

You can't while the guard is there. (Come back later!)

Hint 2 of 8:

That lock on the door seems to be the basic problem.

Hint 3 of 8:

There must be a solution for this somewhere!

Hint 4 of 8:

A strong acid would dissolve the lock.

Hint 5 of 8:

Did you get the bottle from the room beneath the sewer?

Hint 6 of 8:

Did you fill it with yucky green slime?