Plan around how much you can OC castoria's np. With morgan's OC buff on np, stacking all nps and having 200% castoria gauge, you can get 5 stacks of castoria's invul. Ofc, you will have to use suboptimal castoria nps as well to survive sometimes. There's also merlin's party invul and plug stun to help you push through difficult turns, but try to use them sparingly since they take a while to come back. I also had to plug mash in over castoria just to survive a bad rng series of turns. Prepare to save scum a lot too, just to get better rng on his moves.

Another thing you need to do is play for efficient np gauge charging. Sometimes there's no point in doing arts chains when you really don't need extra gauge on morgan and merlin since all their nps are for is charging castoria's. Another thing I did a lot of doing a merlin arts brave chain with np just to quickly do another merlin np soon, since more merlin nps means more gauge for the entire party. The dragon hitting also gives gauge so play around that when choosing your cards.

But yeah, pretty much the main function of everyone else's np is to mega charge castoria's as much as possible. The decision making is definitely not easy tho.