One of the classic ways to be almost invincible to everything is to obtain regeneration, and then make yourself immune to the things your regeneration is weak against. Trolls are a natural choice for the base race here, having regen 5/fire and acid, and half-dragon is the easiest way to get energy immunity, so it's just a matter of deciding between being half-green for acid, or half-red for fire. I chose half-red here, since fire is much more common in my experience. I used equipment to help cover up the acid half.
Because of the synergy factor here, I would highly recommend DMs treat Ixenthrash as a couple CRs higher when deciding to use him, and for XP afterwards too.
Ixenthrash, the Invincible CR 7
Male Half-Dragon (Red) Troll
CE Large Dragon
, Senses ; ; Darkvision 90 ft.; Low-Light Vision; Scent
Languages Draconic, Giant (Does not speak common)
AC 20, touch 11, flat-footed 18 (-1 size, +2 dex, +9 natural))
hp 75 (6d10+36 HD)
Immune Fire; Regeneration 5/acid; Resist Acid 10 (from equipment)
Spd 30 ft. (6 squares), fly 60 ft. (average)
Melee Claw +13/+13 (1d6+10), Bite +8 melee (1d8+10)
Base Atk +4;
Space 10 ft. ft.; Reach 10 ft.
Atk Options Rend 2d6+13, Breath Weapon (6d8 fire, 30ft cone, DC 20)
Combat Gear Cloak of Acid Resistance
Abilities, , , , ,
Feats Alertness, Iron Will, Track
Skills Intimidate +3, Survival +3, Swim +14, Move Silently +6, Spellcraft +2
Ixenthrash is well known by locals living within twenty miles of his hunting grounds for being the biggest, meanest, and hungriest troll around. The fact that he sports demonic looking wings (actually dragon wings) only adds to his fearsome reputation. The fact that fire does not drive him off, like it does other trolls, is another point in its favor. And since he is intelligent enough to have taken a cloak of acid resistance off of an adventurer who once tried (and failed) to kill it, this sole weakness is even covered for all but the strongest acids. Ixenthrash is well convinced not only of his own invincibility, but believes himself to be immortal as well. He goes where he likes, does what he likes, takes what he likes, and is a force to be reckoned with.
Ixenthrash has no fear of death: he will launch himself into combat without hesitation, savagely attacking the nearest target. Ixenthrash has no fear of fire, and will wade right through it with impunity-- even small pools of acid give him little pause, as he places considerable trust in his cloak. He does have some small respect for spellcasters, meaning that if he determines who is casting a spell near him, that will become his primary target if at all possible.
Rend (Ex): If a troll hits with both claw attacks, it latches onto the opponent’s body and tears the flesh. This attack automatically deals an additional 2d6+13 points of damage.
Regeneration (Ex): Only acid deals normal damage to Ixenthrash. If he loses a limb or body part, the lost portion regrows in 3d6 minutes. He can reattach the severed member instantly by holding it to the stump.
Breath Weapon (Su): 30 ft cone of fire, reflex DC 20 for half. This breath weapon is useable only once per day.