I don't think it's broken, I just think it is highly RNG dependent. I was able to finally beat this a few weeks ago, with Mace reinforcing a few times. There is one benefit to this challenge being so difficult, and that is tied to the Chimera. If this challenge was easy, getting Chimera to 7* would be easier as well. Because of the lack of ship diversity Arena parity would lose all viability and one would find themselves in a purely RNG dependent Fleet Arena where the top 300 of any given shard would have the same comps.
I would say they need to add more diversity into Fleet before adjusting the difficulty to the Home One 6* challenge. Doing otherwise would be a poor development decision at this point.