Oh yeah, life crystal farming is a thing now. The current phase 2 ultimate is good for that, since it also drops a sizable amount of gold (16-18k).

But otherwise, 12-5 will be the best farming map in a LONG while. It won't get overshadowed until around chapter 19.

As for event difficulty, there were a few gimmick maps where one random pest/boss would hit like a truck and even full rainbow teams would struggle against them, but beyond that, most full silver/gold teams will get you by just fine.