Pharaonic

For this achievement you need to beat a boss with very low health (your health bar will be flashing), I would guess about 15% or less. I did this against the Ia and Qraqen bosses in the Nile Delta, though I'll admit it was by accident. However I feel this is a pretty manageable fight. The smaller lizard doesn't have particularly high HP, so you can kill the bigger one and then get the small one close to death, and let it get you low and then kill it. For a visual aid, here is a video of me fighting it. I got the achievement from this fight, but ended the video before it popped, since I wasn't expecting it. However you can see from my unlock order that I got them together.

This achievement description is unfortunately inaccurate. What you need to do in order for it to unlock properly is NOT have any Potions left AND be on red flashing health when you get the last hit on a boss.