They will have a cast, or some variation (hold the skill button, or a long animation lock). The last thing we see the Samurai do in the trailer is hold a charge for about 3 seconds before unleashing it. Everything up until that point was a weaponskill (or at least, they seem to invoke the GCD)- You can tell because of the aura coming from the character during activation. The last hit has no ability, OR weaponskill aura. To be fair, there's no cast aura either, but I still think whatever it turns out being, will be functionally similar.