Well, if you can break line of sight (leaving behind some demons so you aren't pursued) you often can escape him and continue exploring- manually, of course. But sometimes positioning and the amount of the level you have explored means it's better to flee, that's all. I've had to flee floors because of simple things like Asrai/Faerie packs that I just wasn't equipped to handle, and Paracelsus tends to have a lot of weakness-exploiting demons so I find him hard to handle with most characters. Paracelsus isn't unkillable- he's just very dangerous, and his tendency to lurk in the back instead of charging out front means you might have to be a little reckless to finish him off.
Not sure how you think it's optimal to lose demons in fights, unless you mean 'let the demon you plan to replace die during a link for a new demon so you save 1 turn not having to unsummon it' but that's awfully specific...