As far as I can tell from my experience with DMM version, it's not that hard to get maxed unit from the event if you spend your time on farming the hardest mission. The real difficulty is... in difficulty of these "hardest" missions, I guess) They are VERY demanding to your team (SSR or UR + solid equip).
Yeah, in FKG you just need to run the same map, like 5-10 times, before RNGesus will have mercy on you, and then smoke bamboo for the rest of the week. Or to make countless auto-runs of the same map (during farm and collection events). While smoking bamboo, of course, because auto-run is just TOO DAMN SLOW.