Tricky -- all the 20-of-everything levels can be solved 100%, and really, any level that gives you enough skills to set and release blockers is no more difficult at RR 99 than normally. So, the only levels I can't see being done are 8 and 22 (seems these are both possible after all!). That's in addition to the levels that are impossible to 100% under any conditions, so the list reads "Non-perfect: 15, 16, 17, 23" (excluding 18 because it wasn't solved in this challenge at all).
Taxing -- as well as the non-100%able levels (7, 19, 27, 28), I doubt the 100% solution to 16 works at RR 99, but 17, 22, 24, 25 are all possible and the rest are easy. So, to clarify, the list reads "Non-perfect: 7, 16, 19, 27" (similarly excluding 28).
Mayhem -- no idea about 2, I can't 100% that one anyway. 7 can't be done. 1 and 25 are possible thanks to steel-cancelling allowing blockers to be released. The rest are easy -- again, apart from the levels that have no 100% solutions (5, 19, 26, 29) (and 10 was unsolved in this challenge).
Frost -- All except 4, 6, 15 can be solved 100%. (6 has no 100% solution. For 4, the only RR99 solution I can find involves making the first lemming from the left group build over the gap, but this leaks one. ccexplore's solution to 15 requires a bomber.)
Hail -- All except 2, 6 can be solved 100%. (These two have no 100% solutions.)