Before using "One Life"; please follow one of the guides below for the best experience.
The "Open to LAN" Method
- Game Mode: Hardcore
- World Type: Biomes O' Plenty
- Generate Structures - ON
- Cheats/Bonus Chest - OFF
When joining the newly created world; you can press the ESC key and press "Open to LAN" click Enable Cheats. Then use this command: /gamerule naturalRegeneration false - Now you can restart your world and open to lan normally.
And, the SERVER PACK method. RUN THE "START.BAT" FILE AND THEN WHEN YOUR SERVER HAS COMPLETELY LOADED, COPY & PASTE THIS COMMAND TO MAKE IT UHC (Ultra Hardcore) gamerule naturalRegeneration false
I hope this helps.