The way to clear the monsters on the first part of the third floor
is to note that there are candles and torches nearby, not too far
from the monster hatching eggs that happen to be sitting next to
TNT.
Light yourself on fire and get to those crates as quickly as you
can in order to explode the eggs as you go.

At the end of the hallway after you have cleared the monsters, you
will see a circle of floating chairs around a table and a door
across the room... this is all suspended in the air above a pit of
death.
The way to get out of this level into the next part is tricky, but
possible.
There's a button in the floor that is depressed every time you run
across it to jump to the chairs... it opens the door across the
room on a timer.
If you take a running jump to a chair it will set the timer, jump
from the first chair to the table, to the next chair and quick as
you can jump to the platform with the door using your spin move
AND float move.
You should just barely zip under the door.

In the next room you will have to jump from garbage to garbage in
the flooded bathroom in order to get to the trophy room.
(I kind of just focused on getting through the room, but I think
there may be more to it on the left side near the leaky
pipes...might be worth checking out if you're brave)