Blazia’s Wrath gives a bonus of 50% of the (cumulative damage / health) of the enemy
So, for example, when the enemy is down to 80% health your shot will do 10% more damage; when down to 50% more health you do 25% more damage, and the max bonus is fractionally below 50% when the enemy is almost dead.
The net result is that you are guaranteed to save a shot only if the enemy needed at least 7 shots to kill (and save 2 shots if it needed at least 12 shots to kill; thereafter the savings is ~1/6 shots), so this is more useful for fast guns (AutoGun / Looper) - powerful guns like the Rifle will still benefit with bosses, and you might still save a shot if there’s some collateral damage (e.g. Zap shots, Death Zap, Death Blast, Fire/Poison etc) or (H/T to vvntn) if you could almost kill an enemy with a few shots (e.g. 45 damage against a 100 HP enemy would normally take 3 shots, but with Wrath can be done with 2)
This only applies to damage from hits, so collateral damage (e.g. poison, fire, flamethrower, any shrapnel (inc. sidewinder), plasma blast, death blast / death zap) doesn’t benefit from Wrath
Wrath stacks really well with Focus Shots