Lilith is a water demonThe most easy way to kill Lilith seemed to be trap her in a oven and set her on fire with no chance of scape. It's obvious that the weakness of something that came from water is fire. And then there's the detail of Emily's aquarium, which the contact with probably made Lilith stronger. That's how she can have managed to survive the house fire, because Emily saved part of the aquarium, including the fish. When they were on the road, Lilith says she hopes they find a hotel with a swimming pool, maybe looking for a bigger point of strength like the aquarium. When the car is thrown in the lake by Emily, she didn't died, she just revealed her true form, because her other one wasn't scaring Emily anymore.
WMG / Case 39