The cataclysm weekly challenge Darkness Falls needs you to complete 10 freeplay anomalies. Since anomalies take 25 minutes to open and the anomaly can be failed if you don't destroy the crystals in time it makes the challenge feel impossible to do, especially solo since there's few people who actually join.
The challenge would be better if it was for attempting the anomalies instead of completing. Otherwise it feels like a waste of time, time that could be used grinding crystals in the cataclysm or completing the daily challenges for war chests and crystals.
Also the anomalies take too long to spawn. It should be shortened to 15 minutes so that they can be chained and so that at least if you fail or miss the incredibly short join window, you don't have so long to wait for the next one.