Threat rating, in singleplayer, is noted underneath the named fighters' name and fighting style, right above where it says "rewards". Only opponents with names have them, so opponents like "dual dagger challenger" don't work.
The first opponent with a threat rating over 100 in the campaign is Onomeus, who has a threat rating of 283, but he's almost impossible to beat with an early character. Instead, move on to the Mercatus district and come back for Onomeus later. After the second opponent in the Mercatus district, all of them will have 100+ threat ratings.
Can probably also be boosted online but it's easy enough to do in singleplayer.