With the Challenge quest - It seems like this game is geared to make you spend more diamonds or energy, or just to annoy you. Case in point:

Under the 'Champions' header, running the XP buff, you're naturally going to see your levels rise quickly. NOT a good thing when it comes to this! Because if you level your characters too high, you'll have to do it again. My Surpreme Galek was around Level 42, and I had already 4 starred my Kael. Whoops! Come to find out, I have to do it AGAIN with another Champion, because I jumped the gun and completed the quest too quickly. 

Same with Missions, ESPECIALLY when it comes to the Arena medals. WHY aren't these quests stacking?? I shouldn't have to farm 20 medals.... THEN a few quests later, having to farm 40! (This is an example). Why wouldn't the 20 medals I had farmed EARLIER contributed to the next quest? 

These are just examples. Are these things intentionally meant to frustrate players?