This event can be completed in less than 45 hours of time spread out over the event's 10 days (if you finish brutal 12.6 very slowly, like 1:40 slowly). It requires less than 2850 green shards. Assuming a level 55+ player using their 125+ energy twice daily, plus the two free energy refills each day from quests and time spent playing (500 energy per day), it requires around 1600 gems for additional energy refills over the course of the 10 days. XP boost must be active for the entire event duration (another 908 gems if one has to purchase XP boosts).
Totals: 2850 green shards, 2508 gems, 45 hours of time.
Until recently, I had nearly 3000 green shards. If I hadn't purchased a couple of blue shard packs with gems, I would have had around 2800 gems.
It can be done. It's not worth the effort, in my opinion.