Probably you saw this video of mine where a NPC rides a MMU/jetpack and is not suffocating.
(sorry for my sloppy English)
This is how I did that:
0) Back up your world.
1) Install NBTEdit mod.
2) Use the command /nbtedit on a NPC
3) Disable "damag"e or enable "invincibility" (you have to search the flag/variable, I don't remember, it was MC 1.7.10, bear me. Ask if you have troubles finding it)
4) Save.
NPC is now invincible. Yeah, that's an all or nothing thing.
(sorry for my sloppy English)
This is how I did that:
0) Back up your world.
1) Install NBTEdit mod.
2) Use the command /nbtedit on a NPC
3) Disable "damag"e or enable "invincibility" (you have to search the flag/variable, I don't remember, it was MC 1.7.10, bear me. Ask if you have troubles finding it)
4) Save.
NPC is now invincible. Yeah, that's an all or nothing thing.