the 12th stage is hard, but heres how i get through usually.
I start the day off going as far as I can,if things start looking bad, i retreat. I spend the rest of the day finishing dailies and other quests to potentially level up or upgrade equips etc. Eventually when there's nothing left to do, im probably a bit strong. and i will make an attempt at clearing what i have left.

I get what you're saying about the power difference. However, a good team that's lower powered CAN beat a higher powered team running in auto.

im sitting at 8.4k power, and i've beaten many players that are a lot more powerful than me. Im talking maybe 10 or 11k power. They're on auto, they will always attack a specific way, use that to your advantage.

My team has won against players more powerful than them since i formed it. at 6.8k power i was beating some 8ks. At 7.5k power i was beating 9.2k. Recently i hit 8.4k and i managed to beat 11.5k power.
Group Convoy, Ranked, Plunders. I've beaten more powerful opponents in all of them. All at least 1k power more than me. It's not easy, but i've won at least 50% of them