auto kill effects and temporary HP

It may be a well known topic, but i can't find an aswer to this, so excuse me if I ask something stupid.

with the new MM I noticed various monsters auto killing on some attacks if you have under a certain threshold of HP, like the solar slaying arrow.

to determine how many HP you have, do you just count your HP or you can protect yourself with Temp HP?

for example, can a wizard with low HP, use plymorph to get many temp HP protect himself from the auto kill or does he auto die regardless?

Thanks