Mine had only 7 health at the end, so it seems unlikely to be relevant
Mine was beat up pretty badly, so I doubt it. IIRC he had less than 10 health.
Con's Bunker did show up in my game, was used, but was destroyed by the end.
Ancient Library was used once by Haka only. Perhaps that is indeed important...
If the rule we're missing isn't that "Haka must have drawn cards via the Ancient Library", my only guesses are that it involves Taiaha and/or Tā Moko. Neither seems that thematic, though.
So I guess the new guess is:
Haka defeats Ambuscade in the Final Wasteland. Other heroes must be incapacitated or removed from play. Haka must have played all three cards with "Haka" in the title (or possibly played at least three) and he must have utilised the Ancient Library to draw cards.
Just unlocked him. Ambuscade in Final Wasteland. Other heroes incapacitated. Played all three "haka" cards. Utilised the Ancient Library to draw 2 cards.
My other thought is number of cards drawn with the library, or how much of Haka's deck you've gone though.
My first game with the above conditions didn't result in an unlock, (all other heroes incapped, library in play, haka wins as last person standing, all 3 hakas used) but my 2nd game did, and it took much longer and I drew a lot more off the ancient library. There were even a few other villain targets in play at the end of the 2nd game, but the last hit on ambuscade got it. I also had savage mana in play and had played enduring intercession, which I hadn't done my first game.
I didn't use the library, it was just there. I was getting beasten up badly enough as it was.
Incidentally, I saw Handelabra tweet some people have won the award for unlocking both Haka and the Scholar, so the information is out there somewhere.
Isn't the award given to the first confirmed unlock? I don't recall there being anything for figuring out the actual condition beyond kudos.
I only used the Library once in my successful run, but given we have one successful unlock without using the Library, I guess that's not one of the conditions...
Could the thing we're missing really have to do with Taiaha and/or Tā Moko?
I hope it's not something easily misconstrued, like "Haka draws at least X cards outside of his Draw Phase" or something.
Or its one of Handelabra's fiendish "do this in an unrelated game, say, lose to Plague Rat to usher in monster world, then do the conditions" unlocks, see GlomWeaver, so we're getting lost.
The Hall of Fame is just for the first confirmed unlocks. Their screen shots of the unlock may provide clues but likely they aren't fully sure what they did.
I unlocked Eternal Haka last night. Played two games vs. Ambuscade in Final Wasteland.
First game (failed unluck) I didn't use Mere, I used each Haka once, but I didn't use the library, and I had Savage Mana in play.
Second game, I used Mere and the Library and successfully unlocked him. I used Savage Mana as the final shot.
I didn't use Ground Pound, Prey on the Weak, or Enduring Intercession.
In both games, I incapped all other heroes (usually self or Haka-inflicted).
In the second game I raced to get Savage Mana out to eat the cloak, but the Final Wasteland pulled Unforgiving Wasteland out first and destroyed it permanently for me. How nice of it.
Hey now, Christopher writes the unlock conditions, so don't blame us 
So now that you've visited the thread, MigrantP, any hints on how close we are to solving this one?
I guess I'll throw one more out there:
Haka is the only active hero and Ambuscade is defeated in the Final Wasteland with Tā Moko in play after having played 3 (perhaps different) cards with Haka in the title while Ancient Library is in play.
I wish I could re-lock him so I can try new possibilities. At this point we're unlocking him on accident almost, not knowing the precise conditions.
I've been watching the thread the whole time. When I see the community come to the correct answer I'll let you know =)
Maybe one condition is to have a victory in the Final Wasteland previously. A lot of people are unlocking him on the second go.
Theory: Beat a villain in the final wasteland. Then have Haka the sole survivor in the rematch?
What if it's we beat MigrantP for the answer! Bats, dual crowbars, raptor bots!
But I got it with Haka at high health, Ambuscade in FInal Wasteland, Haka last survivor, all Hakas used, and killed Ambsuacde with Savage Mana for 80 points (used Haka of Battle)
Doesn't need to be at high health. I unlocked him below 10.
So far the following things seem to be necessary, but not sufficient: Haka sole survivor. Final Wasteland as environment. Play all three Haka cards in the game.
There's obviously another trigger condition, but I think those ones are the only fully common conditions we've had for all unlocks so far. I may have misremembered.
Could they be that the libray and the "unforigven wasteland" in play? Besides for that, my only thing is the order of hakas played?
I think I played Restoration, Battle and Shielding in that order, but I can't remember. Doesn't seem relevant; that's a very secret specific order.
Ancient Library is the only common card for those of us that unlocked it.
OK, I'm going to try something...
Defeat Ambuscade in Final Wasteland with Ancient Library in play. Haka must be on the team and no heroes except Haka have over 9 HP. Three cards with "Haka" in the title must have been played by Haka.