The "Kill X of your opponent's creatures" quest should be replaced with something else. It takes significantly more time/games to accomplish this quest compared to any other quest.

I have attached an example screenshot showing a typical situation. After over a dozen games (with 8 wins) and an 90 + minutes of play time, the quest is still unfinished. And I'm using deck themes with plenty of creature removal to purposefully try and complete the quest more quickly. [[Edit: Added additional screenshot to further illustrate how frustrating this quest is - two more games won and only 2 additional creature kills]]

The major reason this quest takes so long is that it is heavily dependent on your opponents' decks. If you keep getting matched against control decks you can easily go several games in a row with no progress. When you do face an opponent using creatures, the game is usually over with at most 4 or 5 creatures killed, but typically fewer than 3.

Taken altogether, the "kill opponent's creatures" quest becomes probably twice as difficult to complete as any other daily quest on average. It should be replaced with a different quest that is more in line with the other daily quests and is not dependent on facing specific opponent decks to make progress.