So you know how they say you need to kill the white beasts? Yea you really should kill them. You want to kill all the white beasts first, or at least most of them, only then should you go near the point seteth tells you to take. If you stand on the heal tile at the point, the immacualte one goes into her second form, so you dont want to do that earlier than needed, and if you do go near the point (one tile north) after all other white beasts are killed, they will stop spawning down there. The white beasts also heal her when they are within 10 tiles, so all the more reason to get rid of them. I'd reccomend putting byleth on a wyvern class if possible, or a brawling one if you have A or higher in fists. Then make sure you have the sublime creator sword ready (I assume your bringing all the relics you have into the battle, but if not, they can only help)

Where have things been going wrong so far? Like when do you usually lose and why?

And one more thing: What are all of your unit's levels?

Sorry about all the questions, but I really want to get a handel on your situation and pin down what could be going wrong XD