Rain is hard for offensive Steel builds, since you can't outspeed and nothing reasonable resists Water. I wouldn't go with Ferrothorn on a hyper offense team. Z-Happy Hour Jirachi is also still outsped by Mega Swampert and Seismitoad. If you run Calm Mind with Substitute, you still get checked easily by the faster threats of rain.
I would suggest using Autotomize Metronome Celesteela set with Air Slash and Giga Drain, which is pretty potent against Water if played right. Metronome lets you break Toxapex with Air Slash, and Giga Drain breaks almost all rain sweepers easily while giving you healing to get back up to full. Especially since Kingdra is on the down and isn't very used, Celesteela can be a great help.