I believe you guys are thinking of plot 8, plot 7 with Life threatening has Tsunade as a reward for clearing the chapter and the coupons are a reward for clearing with full marks. some of the players I know just beat the chapter first to get Tsunade then come back later for the sealed area when they have enough power and a good formation. I would suggest someone who can spam clones to take all the explosion damage. That's only my thought though. Personally I still need more power to pass it on my alt account I had cleared once with full marks lv 77 and 24k power with a clone spam team and 2 interrupters.