You should have 5 total slots for servants. Use the forced 3 servants up front and throw your own servants in the back line. You can, using only what the story give you, clear out all but the last health bar.

Um....that doesn't make it easy. If your final unit is a particularly strong solo Unit, you'll be fine if you get a single servant brought forward a bit early.

Or, if you have really really strong units, you can throw them up front and ignore the back line of useless servants. But if your front line isn't strong enough, you should throw them in the back instead as the fight is designed around those forced supports and intentionally ignoring the intended function is only fine if you can actually force the outcome you want.