Saeviomagy
Adventurer
With Sly Flourish having a high success rate, and the relative ease of getting combat advantage it seems like a Rogue can deal major damage.
Generally strikers do 200% or more of the damage of other characters per combat. If they only did the same damage as other characters, why would you bother playing one?
Your rogue buddy should be able to do that major damage most of the time he gets a swing in.
HOWEVER
He's very squishy, and in order to do that damage, he tends to expose himself heavily. There will be plenty of times where he's unwilling to get a flank because it exposes him to major hurt, or where he takes the flank and takes the hurting that goes with it too.