I do use waterpump pliers, but the problem with them is that they do not
spread the load on the union nut very well and often the nut feels tighter
than it is because you are squeezing the nut onto the threads

Stillsons of that size I sometimes use but they are fairly clumsy - ok for
system boilers, fairly hopless on most combis.

The ring would have a segment cut from it to allow it to fit onto the pipe,
and then slipped onto the union nut. This would be the most effective tool
at loosening stiff unions as it will distribute the load effectively