Eren and Levi are offering to show her A Cool Thing, banking on the fact that she probably hasn't seen anything like it before (??). Eren will use his ability to turn into a titan (aka 15 meter crazy giant dude) and Levi will then cut his human body out of the nape of the titan's neck, still alive but possibly missing an appendage or two. Levi's part in this depends a lot on the knife that's left in the room, because titan flesh is extremely dense, but if it's just a normal knife he'll stand there and saw him out as long as it takes.

Failing that (and I'm assuming it will fail but hey they tried), Levi will be the one to die.

Difficulty for Levi to cut him out with a regular knife is about a 9 - very difficult not maybe not impossible? If the knife has any special properties or strength it would be more like a 4.