Mamoswine is going to be an absolute force on offence. He will be a dragon slayer. He will outclass the Ice Type by a big margin, and will also do fairly well as a ground attacker.
In regards to Rayquaza he will absolutely destroy him with his 2x weakness to ice
Pokemon Go HUB has him listed in the top 10 counters for both Lati's.
Gunna be one to have multiple of, mostly Ice Movesets but also a couple as ground offence. (Team Fillers)
However the Community Move is yet to be released so all is subject to change, however the move will not change his status of true ice king