How did Lily break down Emily's door if her powers are just illusions.

Easy to explain.
That is if you were going by what they told you in the movie.
Lilith is no ordinary cute little girl but a demon in human form,
similar to that demonic Damien in the Omen movie.
Therefore it goes to explain her supernatural powers and unsettling way
of altering her voice in unnatural non-human manner.

No her powers were no illusion in this movie.
Also it's no coincidence that her name is Lilith?
A female demon from Babylonian and Jewish myth.

Maybe this world is another planet's Hell. - Aldous Huxley (1894-1963)

Uhhh, her body transformed into demon beast, which she could do for short time. Watch the film, really watch the film, no looking down in popcorn bowl.

She never changed, it was all illusions ffs!

The door wasn't smashed, and she never entered the room.

See when Emily is chased by Lilly's burned mother. She pretends to be in her closet, Emily gets scared to death. But in many quick scenes, Emily is trying to flee and there's NOBODY in the bedroom! It ends up Emily herself opens the door!

And when Lilly's mother leaves the house she breaks that wall, but next day the wall is back there!

What I don't understand is how Emily left the room after the door was smashed. She'd not open it because in her illusion it was broken, but it was there, and to open it she'd need to move the bed, the cabinet and open all locks. Would Lilly be able to make her do all that without noticing?