What's the point of walling suicune? I want to kill suicune. If I can't kill it than it can take its time killing me.
Maybe if you're running blissey or celebi, but for most sweepers a stab surf is going to leave a nice 30-40% sting at least even if coming from low special attack. Considering how few pokemon actually resist water, it's a (and that many non-water OU pokes who do are in turn weak to SR), along side all the terrific non-offensive options boasted by bulky waters, and they pose a big threat.
Besides, even if you are something not killed off the bat by surf, it doesn't mean you can beat suicune either. Also none water pokemon who resist water, hate this thing called "ice beam"?
30-40% would generally be done by a Modest Suicunce with 252 SpcAtt EVs, which is a waste of walling capabilities. You could have least mentioned Vaporeon, who has base 110 spcatt and could actually reasonably that get damage (but still not to your standard). Vaporeon can 2HKO lucario, who has low defences. Skymin's are higher and has a resist, she will take less than 25% guaranteed.
And there are MANY water pokemon in the metagame, almost every team has at least 1 - gyara, swampert (with earthquake for other waters to neutralise his 1x water resist), starmie, suicune, vaporeon being the main ones.
Of course, Skymin would be a dumbass to swap in on Suicune (unless he can OHKO) thanks to Ice Beam (unless he's your. But Suicune is also a dumbass to swap in on Skymin, whose only hope of OHKO is hax - seed flare miss.