I got pretty close with Nightmare, but I highly recommend Ivy.  Use her 6+Y (forward + Y) move, which shoots her whip straight out.  It has incredible range and opponents almost never block it, though if they jump back at the start of the match then you will miss.

Here is a technique I discovered while playing which led to my getting the achievement.  At first, I would just try to pull the move at the start of the match, only to have opponents evade it by jumping backward, and then run at me and hit me with a long range move before I could recover.

So, instead what I did was to move backward at the start of the match, all the way to the edge of the ring.  Then I would just wait until the opponent charged at me, and because s/he would always be far away, I had plenty of time to decide when to use the 6+Y move to stop the opponent in his/her tracks.  There were one or two times that I did not have a chance to get that far away before the other guy charged, and in those cases I just pulled the whip right out.  The achievement did not unlock immediately; it only unlocked after I was beaten, so don't worry if it doesn't pop right when you get the 25th win.

Use caution with this technique if you are playing against Taki or Yoshimitsu.  She can jump in the air and divebomb you from quite a distance.  It may be better to just try to take her out as quickly as possible.  Yoshimitsu can teleport to right behind you, although I don't know whether the move is limited in distance or not.  I never had him try it when I was far away from him; when he did do it, he would for some reason not attack me right away after teleporting so I'd just hit him quickly.