I've been playing this game for 6 months now.
The first couple of months I had no chance of qualifying for the 3rd reward (20 gems or level 5 health rune) in the timed challenges.
Then for about 3 months I was able to qualify in collecting gold, feeding monsters, and acquiring runes.
Over the last month the requirements for these challenges seems to be going up exponentially.
For example, a month ago I only needed 700 points to get the level 5 health rune in the rune challenge. I was able to qualify for this fairly easily. Since then it has gone up to 2300 points last week, which I have no chance of getting without spending gems. Strangely, the "grand prize" is always a level 5 health rune but the requirement to get it has more than tripled! One would think that if I need to collect so many more runes now, the reward would also improve?
The same goes for the gold collection and monster feeding challenges. Just last week I needed to collect 75 million gold to qualify on the gold collection challenge--double what it was a month before. This week it is up to 90 million. I had no chance of getting the 75 million last week and I have even less of a chance of getting the 90 million this week.
So how is this calculated? I haven't been rocketing up in level over the last month. As a matter of fact it's just the opposite. I've really slowed down my leveling after reading on this forum that you get penalized for leveling up. Is it my level? Monster power? Something else?
I just can't figure out why the challenge requirements have gone up so drastically just in the last month to the point where I have zero possibility of getting the best rewards?