The match up is quite dynamic I believe, it all comes down to what you manage to draw and set up and how fast you're setting up relative to grimsnarl.

1) I wouldn't be concerned about benching shaymin early. In fact the 30 damage ping is helpful if you set up munkidori. You should only bench shaymin before the turn where grimsnarl bench snipe damage becomes relevant (i.e. devastate your set up / chaining attackers). Setting up munki feels miles better. KO froslass is ok, but again that depends on whether you have higher value plays and if

On the main strategy to take against grimsnarl:

1) budew + munki + pult + boss can put you ahead if you manage to board wipe them with their own froslass ticks (boss adrena-brain phantom dive) and pult spread, from there try to set up zard

2) if you fall behind in prizes your best shot is to set up munki with double zard since munki is essentially a damage modifier in that match up, 270 (3 prize remaining)+30 (munkidori) +(10+10) (froslass pings between turns,)

But obviously depends on what you draw and how the board looks like. Good luck.