Which is the highest level Pokemon used by a in game trainer?

Question

I want to battle him.

Answer 1

The strongest trainer Pokemon in all games is Red's Pikachu in HG/SS at Lv 88. The rest of Red's Pokemon range between 80 to 84.
Source: http://bulbapedia.bulbagarden.net/wiki/Red's_Pikachu_(game)
http://bulbapedia.bulbagarden.net/wiki/Red_(game)#Pok.C3.A9mon_HeartGold_and_SoulSilver

Comments

the 2nd highest after red's pikachu is Barry's starter in pokemon platinum after beating the elite four 20 times , his pokemon is level 85

Answer 2

In Black 2 and White 2 there is a list of strong trainers.

[Here][1]

So the simple answer is Cynthia, Game Freak Nishino, Game Freak Morimoto

//Jofly
[1]: http://www.serebii.net/black2white2/strongtrainers.shtml