you'll need to jump and hit the slime with a basic attack, then jump to do the roll action (not really sure what it's actually called), then dash into the room.

if you can't get it, just practice jump-hitting the slime and rolling while standing directly on top of it.
once you get the hang of it, it should be fairly easy.

this also works for getting on the roof of the main entrance for the "fresh air" achievement.