First, I would go with the ECL's from Dragon #293. That would be 11 for being troll and +4 for being half-ragon. So he would be CL1 when the rest of the party is CL16. And he had to face CR 16 Monsters, or a CL 16 Character!
He would get 6HD for being Troll. Usually these would be 6d8, but I think the half-dragon will change that to 6d12.
Now for being "invincible": He is a munchkin, and therefore he can expect to have to fight another munchkin. Consider a Mage or Sorcerer of his ECL (if he only has 1 Class Level, this would be a Sor16 or Wiz16, usually an Evo16). He would have an Intelligence/Charisma of 20 (at least: we're talking of Lv 16, so he would get 4 points to boost his INT/CHA, and much mony to buy magic items to further power it up), Spell Focus (Evocation) and Greater Spell Focus so his spells are pretty hard to save against (19 + Spell Level). He'll have energy substitution (Acid), Energy Admixture(Acid) and Maximize Spell.
This Evocer could deal considerable acid damage to the Dragontroll: a Maximized Acid Orb (Spell Level 7) will deal 90 points of acid damage (Fortitude DC 23 half), an Acid Admixed Acid Orb (Spell Level 8) would deal 30d6 points of acid damage (Fort DC 23 Half) for an average of 105, an Acid Substituted Maximized Cone of Cold (SL 8) would also deal 90 points of acid damage (Ref DC 24 Half) and since the wizard is hasted, he could cast two such spells in one round! Such an Evocer would have 4 7th-level spells and 3 8th-level spells, so he could do this three rounds long before resorting to "lesser" spells, as Maximized Acid Substituted Fireball (60 Damage, SL 6) or such. Of course, he could also have a rod of Maximization (maximize spell without hightening the spell level 3/day), either normal or greater, which would allow to maximize the Acid Admixed Acid orb, which would then deal 180 Acid Damage. Thrice (twice in the first round, another time the next before the rod is useless for the day).
Also, a character with 16 levels in a battle-oriented class would just hack him to pieces and drown him in acid.
So invincibility is far away if you "fight fire with fire" (one munchkin with another...)