Niblog
Its a CR 1 creature so it can only play at level 2 or higher. But its special abilities makes this an insanely powerful Monster and even more so a Player Character.
The only way to really damage the Niblog is by using curative magic or effects. So pretty much pour a healing potion down their throat, or cast the Cure X Wounds on them to take them down.
But it is still capable of being hit with spells that cause other damage aside from HP damage. So I do not view this as much of a problem as it first appears.
So what class would a Niblog just mesh with perfectly? I ask because I am possibly going to make one as a PC but mostly I want a real pain in the ass go to option for a Goblin leader.
I do know they have that Confusion ability but the DC is 11 which is easy to meet even with a +0 to Will saves
First, it's not Paizo.
Second, be very sure your GM is okay with your choice. I mean, very very sure.
Third, I'm pretty sure your GM will think at any trick to harm you. For example, resting recovers hp, but for you, it becomes dangerous.
Fourth, I suggest you to play a monk. Regardless the synergy, monks have good Touch AC, and if you are going to get harmed, 90% of those attempts are going to be touch attacks. Channel energy becomes painful (but you'll have high will save), to a lesser degree Lay on Hands.
Fifth, before even sitting at the table, define with your GM what "death" means. Can you be "hitted-to-life" if you reach your negative constitution score? Resurrection works? Ask then play.
Fifth, here to you some hints:
1) Coup-the-grace yourself to full heal.
2) Explosive-Rune yourself, fail the dispel check and be a kamikaze.
3) Lava becomes easily "swimmable".
4) Ignore negative channelers.
5) Stop eating, find a way to ignore fatigue and exhaustion and reverse regenerate yourself.
6) The best way to win is to be inside an Antimagic Field. You are utterly unkillable, as only magical healing works on you.
Yeah I didn't think of that when I posted.
Having a Low AC is not too bad unless it comes to spells that have a save of die effect or do not deal HP damage. The fact that sleeping can heal you is a little interesting but it specifically calls out damage not healing, so I think it only means positive energy style of healing.
Your suggestions are good ideas.
That is true, there are ways to fight those who heal from HP damaging attacks.