Re: Scyther with eviolite
I don't believe in Scyther, with or without Eviolite. His typing is absolutely horrible... weaknesses to fire, ice, electric, flying and 4x rock is really bad, though 4x resistance vs fighting is very good in current metagame. Also, you lose a Stab priority move, along with 25 base attack. Also, Scizor has 20 more base defence so half of your Eviolite bonus doesn't really matter anymore.
Roost can only be obtained by TM in gen IV. If you only have Black/White, u can't get it. However, I think there gonna be movetutors in Pokemon 'Grey' so maybe one day u can still get it. Also, I see some people here talk about Aerial Ace. It's pretty good with Technician Boost (135 base) which is the biggest plus about Scyther, alongside good speed.
Whatever you do, make sure not to use X-Scissor but Bug Bite because of the Technician boost. Also, I strongly recommend Swords Dance, Jolly nature and maybe even Focus Sash... good luck!!
Black Trainer Card in Pokemon SoulSilver