Easiest way to do this is to play as Toro or another character with a one hit Level 3 (where it insta kills everyone), or play as Raiden or someone else who has a Level 1 or 2 that is easy to get three kills with (usually a melee Level 1 or 2). Just build up meter, don't worry about trying to get kills, just stay alive and build meter until you hit the level that you need for whichever character you're using. If you're good at the game, you could attempt to use a Level 1 or 2, however the safest and by far easiest way is to go for the one hit insta kill Level 3's that some characters have.