Before enter to the world boss how to know health of boss, when I enter remain 8 tube my loot so sad

First of all, there are multiple channels for Kzarka. Not everyone goes to the same one. Every channel has Kzarka at a different HP. With that said, there is no way to tell.

Try joining when he first spawns. Ive always gotten x20 health bars that way. Its usually when i join late its otherwise. Im lucky if i even get a purple item though anytime at lvl 54 and 2500cp. But since pearl abyss added the refined blackstones i get those too now. You might need to be at a much higher cp and damage output if you want to get those epic and mystical drops.