Like a lot of the recent Legendary Quests, the latest one I got, A Worth Opponent, can be completed by playing other games than the description described. This one requires us to play two games in Play Mode. I just played a game in Tavern Brawl and got the 1/2 credit (I was at 0/2 before the game).
https://imgur.com/o54AOzU
Edit: Friendly duel also works (I did a Wild duel for this test; my friend initiated the duel).
https://imgur.com/ymaHe6M