I've done quick calculation for this extremely boring event (even Peric's favorite Air Ballon was more fun...) and here is result.

First to mention - it doesn't matter which level you choose, for same amount of energy you'll get same xp in any of them (higher levels only requires less clicking).

Your max energy starts at 31, so you'll prob reach  41 in couple days. I'll use 35 as middle value.

If your day cycle is 7 hours sleep and 17 hours active, you'll have 35 energy at the start of the day.
35 energy equals 525 xp.
In 17 hours you can restore max 17x6=102 energy
102 energy equals 1500 xp

Hence for this example you can get max 2025 xp daily. In 21 days of event it is 42525xp.
For last reward you need 43 levels - so 43000 xp.

Conclusion: You need to sleep less, and never let your energy overflow your max amount and waste it 


PS: When you reach 40 as your max energy, you'll have 75xp more per day.
So in theory you can reach last reward, but it is highly unlikely that someone will not be away from the game in more then 6-7 hours at once in next 21 days (so you energy is not wasted) ...
That is reason why admin gave us option to buy 100 energy (1500 xp) for 150g, but it is prob better to save gold for pink stone, unless you're short for small amount of xp on last day of event.