Lemnis Gate

You can boost this with a friend on normal (as opposed to ranked) matches which still count as online play. Just play any of the 1v1 variants and line up at least three characters together without moving and have your friend do the same. Afterwards, it’s just a matter of both using Striker and his ability so you have enough time to get the shots in real quick. The achievement popped instantly for my partner and me. You can use this method for most of the “online match” achievements. Just make sure to not move the characters around much so that you won’t have to chase them during the Striker turn.