This is best boosted with a partner as this game is quite dead as of the 15th of July, 2014.
If you don't boost, you're probably going to have a tough time finding anybody to play against to get this achievement. If you do manage to find somebody online, I suggest you use either Faust, Zappa or Millia as they're generally considered high tier fighters.
Whether you're boosting or not, set up a Player Match of 1 Round. Use instant kills to fasten up all 200 online matches (minimum) you'll need to play if you plan on completing the game / online MP.
To perform an instant kill, first press A + B + X + Y at the same time. Your character will now have a glowing outline. Then either use the control stick / D-pad as you press down, then toward your opponent, then down & finally hold B whilst pressing toward your opponent for the second time. Basically; press down, toward opponent, down, toward opponent AND B. I was playing as Sol-Badguy when performing instant kills. Every character uses these commands for an instant skill except for May, Dizzy & Order-Sol.
You'll get used to the timing even if you don't usually play Fighters.