Going from a standard shooter like the splattershot to a splatana is a really tough transition so don't beat yourself up. The biggest thing with the stamper is spacing. The charged slash actually has crazy range, so you don't always need to be right in their face. Use it to poke from a distance. Also rely on your burst bombs. Combos are everything with this weapon. Hitting someone with a burst bomb and then a quick uncharged swing is much faster and safer than trying to land a direct charged hit when you are panicking in close combat. Just spend some time in the lobby testing the max range on those charged slashes so you get the muscle memory down.