I got the achievement today in ranked standard 1 : you need to reach 50 before killing your opponent.

Anything that happens after your opponent reaches 0 doesn't count : so the lifelink from an attack that kills wont trigger the achievement.

I got the exact same bug : I just finished a starter deck event duel with 60 life and no achievement.

I just finished a Starter Deck Duel using Vampiric Hunger where I finished with 66 life thanks mainly to Elenda, Saint of Dusk, and my opponent being nice and not conceding early, and... this "Test of Endurance" achievement did not trigger for me.

I had this issue with "The Only Point That Matters" recently too, where the first time I won a duel with 1 life left, it didn't unlock, but it did successfully unlock the second time I won with 1 life later. Why are some achievements being inconsistent with whether they unlock or not for me?

(And, there's no disclaimer on the achievement saying Starter Deck Duels don't count, and I've gotten other achievements and completed daily challenge quests using Starter Deck Duels before.)

Just happened to me as well. Wondering if you also have to win the game. As soon as I hit 50+ I conceded so as to not waste any more of the random opponent's time. I would have killed them within a couple turns, but still.

I've reached 50+ in a game now and the acheivement hasn't triggered (Wasn't in a direct challenge nor against Sparky)