Campaign is most efficient by far. Event maps are only good for getting 3* clear bonuses and if you need the specific items (specific manyu/dragons, and some events are good for grinding life crystals).

If you struggle at the highest available story map, I recommend getting manyu and/or dragons as needed.

edit: If you struggle to 3* maps not because teams are dying but because you're not able to reach some enemies, play around with where you start your teams. If it's because they're dying, switch to your second set and manually set your teams so there's not such a big difference between your strongest and weakest - then go back to the lowest map you haven't 3*d, get a 3*, and move on until you've gotten all 3*, while aiming for hidden stages and more manyu.