The third one because the boss there only has one health bar.
It’s much easier if you start your run from first biome and then get to 3rd one before attempting this.
You will be buffed up and all enemies will be melted easily.
if the power cut is frequent, then yes, it's advisable to use UPS. i haven't used UPS my entire life, nothing happened even when there was a period of cut off's.
if u use low quality power supply, that is bound to cause an issue, same goes with motherboard.