The top 100 or so have moved on from this game to The Broken Bond, making this elusive achievement much easier to obtain without getting beaten a lot. To get this achievement you must essentially win 5 games in a row in the online tournament.
Orochimaru is, ironically, the worst character in the game by miles. But he does have strengths to play to.
First of all, use the YXX combo often to get enemies into the air, A+X to send them across the map, and get the highest level of Shadow Snake you can. Practise this jutsu in single player until you get very good at it, if pulled off perfectly, it can nearly kill your opponent in one. Don't bother using Genjutsu, Down+Y, Up+Y-Y or your other killer combos, they're useless and predictable and will only led to you getting side-stepped then thrown. Few taps of the X, Y, Down+X, side-stepping etc. alongside the method mentioned earlier when fighting will lead to you dominating the matches. Practice makes perfect, there really is no shortcut for this or any of the other Forest of Death exam achievements.
PS. Remember, low hitting attacks can be blocked; and substituted! Use Down+B!