Generally, when using a non-Uber/ non-Steel, their role is to provide a nice within which to exploit one common pokemon. Scarfape's main role is to keep Darkrai in check, using it to gain an advantageous switch. In a pinch, it can be used to remove a sDialga at reasonable HP, or T-Tar, but it should not be used as a counter.
Similarly, you don't chuck Quagsire/Ludicolo on a team for no apparent reason. The sole reason its there is to force Kyogre out, and once your opponent is aware that it is present, it will force them to make certain moves, easing prediction.
As for Mew, I find Explosion is also a reliable way to remove it. Also, a slower U-Turner Baton passer (which shouldn't be too much of a problem if Mew is RPing), can be used if you predict the switch and go to your designated counter. For example, I see Kyogre is a pretty popular Rock Polish Recipient. Scizor pressures Mew and when I predict the BP, BAM, go to Ludicolo. Alernatively, you can always just try a scarf taunter like Mewtwo/Darkrai. Assuming Mew is fully Bulky, you can actually stop the pass, but thats gimmicky at best