This one is actually not so bad since you only have to strike 2 enemies as most of the class based achievements. I find easier to do it on a 2v2 match. Select the extended range modifier for this class and start a match. When you're playing on a stage with the stun upgrade (the little lightning icon that appears on the fllor) keep an eye out for it. If you're playing against the AI your opponents tend to be near each other by the time the stun upgrade appear so make sure you reach it before the AI and activate it. The effect of this upgrade is very short so upon stunning your opponents immediately run to them and strike to claim your achievement.
If they are too far apart wait a little bit until they are closer to each other, just enough for you to kill both of them with a single blow, just don't wait too long or else the stun upgrade will disappear. You have a few seconds to collect it. I know it sounds challenging and tough but it's not. Like I said, if you're playing on a stage with the stun upgrade you'll probably get it no time after a few attempts. This also can be done on a stage that does not contain the stun upgrade of course, but it'll be harder. Since you can't select the stage you're playing I recommend you set up a 2v2 match and start working on the achievement. Try your best to pull it off and if you get it, GREAT, but if you're feeling it's too difficult keep playing until you get a stage with the stun upgrade and you'll see it'll be much easier. Good luck!