I must confess, I was so frustrated with Magni during the preview weekend that I got this build from Guru (for shame), but it does work against everyone except Gwen (although I have beaten her with it twice, but that was more down to luck than anything else).

Skills
Critical Agility (Sunspear)
Critical Defenses (Critical Strikes)
Way of Perfection (Shadow Arts)
Beguiling Haze (Shadow Arts, Elite)
Golden Fox Strike (Dagger Mastery)
Wild Strike (Dagger Mastery)
Critical Strike (Critical Strikes)
Impale (Deadly Arts)

Attributes
Critical Strikes 12 + 1
Dagger Mastery 10 + 1
Shadow Arts 7 + 1 + 1
Deadly Arts 4

The above is a much messier attribute spread than I am usually comfortable with; however, the deep wound from impale does a really nice job of finishing your opponent.

Equipment
Radiant Armour
Runes for the attributes as stated (a little alteration here shouldn't make too much difference).
Silencing or Zealous daggers with a 15^50 inscrïption (Droknar's Knives, end game GW:EN item is great for this).

Useage
Before you go in to attack, apply all your enchantments, shadow step to your opponent using Beguiling Haze then use skills 5 to 8  (with some opponents you will need to do this twice). The Dazed condition on your opponent will mean you interrupt just about everything. You should get through rounds 1 to 5 with little trouble (unless Gwen is your 5th competitor), M Bison himself is a little more challenging - you must make sure you keep up critical defenses against him in order to win.