Pokemon Battle Question, Weavile, expert opinion needed?

O.K. So my friend challenged me to a Pokemon battle. The rules will be level 55 one on one, and one non legendary Pokemon allowed. I have decided, based on my strategy, and what I know of him, to use a Weavile. However i have a dilemma, I cant decide weather to give it an attack or speed boosting nature. Normally i would go with speed, but in this type of battle i only have to face one Pokemon, so i am not sure.

If i go with speed, i would still have to worry about the possibility of a speed trained Ninjask, Electrode, Aerodactyl, Jolteon and Crobat (and Swellow, who is as fast as Weavile), as well as reduced attack.

If i do go with attack, however, I would also have to worry about Dugtrio, Alakazam, Sceptile, Starmie, Floatzel, Ambipalm, Gengar Tauros, Espeon, and Froslass, but I get a major boost in power.

I cant decide which to do, I would usually do speed, due to these risks, but in a one on one, i would expect something more like a Dragonite, Tyranitar or Metagross from him, in which case i should go with attack.

So, i want to know what you think i should do. Right now i am leaning towards attack, but i want some outside input before i decide. Thanks.