Well I had to use 4x bloody armors for all the super boss fights and Live flayers on earth mode I always took the full 9999 damage per hit against all those and still never had guts kick in.
So when it comes to "breaking" the game using 4x bloody armors to avoid all damage while causing 9999 per hit is the best and cheapest way to do so.
Live flayers multi-hit you with their spread shots if you're point blank, which means its not 9999 in one hit, but more like 2000 damage x5, which is why guts doesn't kick in. The alternative to using bloody armor, is simply bring Rena along and have her use star flare to hit stun them all so they can't just spam shoot you non stop and interrupt your attacks.
Claude Opera or Ashton Rena Dias (player controlled)
All Claude needs is ripper blast, Opera - homing energy shot and healing star. Have Rena AI set on "Do what it takes to win" and enable star flare. If you're using Ashton, all he needs is hurricane slash, and tri-ace (if you got it) As the player, just spam air slash from a distance with Dias. If someone dies, use Rena, or Dias to use revive mists.