Whether you roll the gacha one or 10 at a time doesn't matter if you are trying to clear all or most of the prizes on a given gacha page. If you want to reset with more things left then its technically better to roll one at a time as long as you are willing to suffer through the increased animation time. Judging from the guide on Discord, the special farmable stage in phase 2 will be a 700 sunshine for 20 fireflies, as opposed to 345 for 10. Slightly better but I wouldn't sweat it too much either way.

Personally I found it tricky to three medal E-4; I had to go in with a distinct speed imbalance and proper placement of the groups to make sure that both bosses got killed. That took several tries.