From the official Supercell forums:
Yesterday was quite busy and I wasn’t able to update you on our current thinking about Dr. Terror, so here it is:
We’ve seen the server stats and now know how many people are able to beat Saturday Terror and Wednesday Terror. He’s close to the difficulty we want him to be at, as clearing 20 stages is intended to be very hard and only possible once you reach the end game (max troops, etc).
However, his Wednesday event is a bit too hard right now, considering you unlock it before the Saturday event, so we’re going to reduce the difficulty of Wednesday Terror. We’ll probably lower the building health statue percentages to achieve this. I’m not sure by how much yet, as we need to test it some more, but the plan is basically to make Wednesday Terror a bit easier to complete than Saturday Terror. Although, the random generation of Terror stages can make this a bit of a challenge, so balancing it isn’t going to be an exact science. Once we’ve made the changes, it will be great to know what you guys think and whether Wednesday is indeed a bit easier than Saturday.
In addition to this, we’re going to add one (extra) guaranteed Dark Crystal to stage 19 (thanks to LANfill for this suggestion), so there will be three guaranteed Dark Crystals in total for clearing all 20 stages, on top of the usual random drop chance Dark Power Stones. This will make the Power Powder expenditure more worthwhile and compensate for the extra difficulty.