This literally means getting the first attack (confirmed hit, not blocked) in a match, either in Arcade or online.
Easiest way is to play Arcade, setting difficulty to "very easy" (hit
on character select screen for options) and you can just use a hyper combo right when each round begins (
+
works for every character). On very easy, the AI will rarely ever block it.
Another easy method is equipping the red Reality stone and hitting
as soon as each round begins -- this will fire a homing projectile.
Make sure you actually finish the matches for them to count; also the achievement won't pop until the end of the match where you got the 50th first hit.