At round 2, the Kyuubi will use either Exploding Hell to do great damage to your whole team or Roar to reduce damage of your whole team, ang it's random.
At lower level, you may hate Exploding Hell but once you can survive it, you will like it more than Roar

Is it defense, resist or both that lowers this specific attack?

I don't think it's random - somebody I know said you last longer if you cause more damage; that is, Exploding Hell is an indication you haven't done enough. And even if that's wrong, you start surviving Exploding Hell around level 50 (I was so happy! I mean, he beat me later, but...).

What's your team?

You can survive exploding hell at 40 with Anbu buff.

It's related to the world boss being strong, and you're not strong yet, most people end up running back after dying 20-30 times before much higher levels.

*edit* urgh crap can't delete this to merge with previous post lol

It's actually not as random as people think, the more clones you have the higher chances of him using exploding hell, I used 3* Naruto, Wind main making clone each turn, and 1 more character making clones (a single clone), Kyuubi would use it almost every turn after first.
So lower your clone number to lower his exploding hells. This post was last edited by 101***@facebook at 2016-8-27 10:23

Granted I started very recently, and I'm still getting my team right. I'm over 9000 power right now, though.

I'd like to spend coins/coupons but I donate to my group a lot so I never have enough money by the time the event rolls around, and I try to save my coupons. I think I did Hokage support once and it bumped me up a bit.

I have up to four clones (Kabuto, Kiba, Kakashi) and it doesn't seem different than when I had my Anko/Kakashi/Gai team...

I tried running with Naruto (3 clones), water main, Lee, Tenten. i got around 2k combo and ended up rank 22 by the end (without anbu support), it used exploding hell only about half of the time this time

and since i came prepared with very high focus on def and resist i was able to survive even when he used it (most of the time) with all characters as a level 43.

I still cant say for sure, but the damage done seems to be a factor in terms of how OFTEN he uses it at least, still seems random, but very much less this time.

Ninetails things you need to know:

1. If he uses exploding hell, be glad it wasn't tailed bomb or Roar (Not way to deal with this besides getting stronger and leveling.

2. Do dps - Use a 4 type team, barrier etc, for example waters should use something like:
Main-- - Haku - X
Tenten - X ---- X
Suigetsu - X -- X

3. You're going to die...a lot.