i summon a zombie with 100 health
(summon Zombie ~ ~1 ~ {Attributes[{Name:generic.maxHealth,Base:100.0}]}) ,
it took quite a while to kill it with a sword, then i summon a new one, then i take a gun it dies when i deal 20 instead of 100 damage (gun damage is 1). I used damage indicator plugin to saw this.

Why does this happen?

(sorry for the bad english)

I don't understand the issue here...

A zombie with 20 health (10 hearts) dies when you deal 20 damage with a sword or a bow, if you summon a zombie with 100 health (50 hearts) it dies when you deal 100 damage with a sword or a bow, but when using a gun it dies when you deal 20 damage and not 100.
I am making a game so thats a big issue! No matter how much life points i give them, they still die in a couple of shots.

I hope you understand now.

sorry we think this is a long time bug that is based on an internal limitation on bukkit damage system. unfortunately, we found that monsters like super big slimes and spawned boss monsters were getting killed really easy by bullets and even a stack trace sometimes would appear. please let us know if yo usee a stack trace, but this might be a long time issue