MobHealth3 Kronos Edition 1.12 - Shows the exact NPC Health. (Extracted from vanilla-twinhead.twinstar.cz).
Step by Step:
Download newest Version. (Clone or Download Button)
Drag the MobHealth folder inside of MobHealth3-Kronos3-Edition-master into World of Warcraft 1.12InterfaceAddOns.
Download: http://bit.ly/KronosDBMobHealth
Put the file from step 3 into World of Warcraft 1.12WTFAccountAccountNameSavedVariables
Done
Standard Blizzard UI User also need: https://www.wowace.com/projects/project-14403/files
Example

Git: https://github.com/Paparogue/MobHealth3-Kronos3-Edition