Originally Posted by Nanaya

Which avatars? If you mean the dark imperial set (name varies a little by class) you just buy bind cubes and use them (cash shop, click the coin looking icon at the top and it's on the second page of one of the sub-categories; btw, thieves cannot buy bind cubes at all.) As for which avatar pieces to place into the cube, it has to be 2 of the same gear-type (hair+hair, body+body, etc) and you should use the permanent avatars instead of the time limit ones for this. The last thing you do is pray you get lucky.

There's an avatar guide floating around these forums somewhere that tells you which avatar piece has which available bonuses although I haven't seen an attack speed boost bonus (there is a cast speed one but I doubt it's the same thing.) Maybe the attack speed is from a full set bonus or something. The only other thing I can think of is that those are event avatars and we have a snowball's chance in hell of ever getting one.

EDIT: Forgot to mention that you can only get those avatars by setting the bind cube to random.