I've ran Beginner, Standard, Expert once each just to get the 50 Jewels x 3. I've not missed a single day worth of Ragnarok. I've spent about 40 half elixirs all on Ulti. Along with natural recovery, I think I've defeated Ultimate for about 60+ times.

Out of all these, I've seen 3 rainbow chests that are not Ragnarok core rewards. One Yatagarasu, one each of the SR exchange weapon except for the bow. So the 2% drop chance is about right.