How would I make it so that if an NPC's health reaches 10 or under it automatically refills fully? Like what would I put in the script?

Any reason you don't want to just give the NPC an ability that restores massive amounts of health every second?

Never though about that. How would I go about scripting that?

No script is needed. Just make a spell that restores ~1000 health, select all the necessary option to make it an ability (there should be either a check box or a drop down menu), then give it to your NPC.

if you're looking for a script like that, check out the script on dagoth ur