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.

Then I as I said I don't know what to tell ya it will obviously take more power then what you currently have or you can try checking other threads like these:

http://forum.naruto.oasgames.com/en/forum.php?mod=viewthread&tid=6173&highlight=life%2Bthreatening%2Bbattle

http://forum.naruto.oasgames.com/en/forum.php?mod=viewthread&tid=6612&page=1#pid48008

well there's my input if that doesn't help wait and get more power try farming in the eight inner gates you'd be surprised the difference it makes.