Of course if you DON'T have/know how to make a virtual HP system, here's a few suggestions---
For Detecting:
-Use EUD conditions to detect the boss's HP (It may only work if he's preplaced)
-Use an amount of time through the battle instead of 50% HP and set him to 50% HP.
-Give him 1/2 of his health, then when he dies have him use DoTs and make another one of him.
For Poison:
-Have a cloaked mutalisk or wraith above the person that slowly damages them and run an AI script where they upgrade their damage
-Have him activate lurkers in the arena. They might not look like poison but spikes are a nice effect too.
-Put the player near a defiler/science vessel that plagues/irradiates them, keep in mind you might need a low health high armor alternative for this, seeing how plague and irradiate both do low damage.
Without virtual HP you can't easily make something that looks realistic unless your creative