Spartacus Legends

I didn't get this achievement until I reached the mercatus district I found a fighter who has exactly 100 as his threat rating and luckily for me I took him down fast.

For this achievement I recommend fighting Gracilius the Gutter in the Mercatus district. I used twin daggers to beat him since they made me very fast and agile compared to him.

He isn't too hard to defeat (at least in my case he was not) just keep the pressure on and he will crack after a good beating, good luck!

Video below showing his location, the fight and my using twin daggers to take him down. He was the first over 100 threat enemy I have found so far.

Just got this achievement.

I used the dual dagger character I had to unlock for the fight before Oenemaus and had him set up to a 68 rating. Oenemaus has a rating of 283 and is the legend fight in the 1st area. To unlock this fight you have to beat all 'named' challengers leading upto it in that area.

I equipped the better armours I had unlocked and also equipped the perk I unlocked in the previous fight which gives bonuses in fights against shielded opponents (which is what Oenemaus is).

When in the fight, quite boring but I just mashed light attacks. He takes half of your health with a small chain of his attacks but hopefully you will be quick enough with light attacks to counter each flurry.

It took me 2 goes, first fight was close but lost, the 2nd fight he only hit me with the one flurry.

You will also unlock 2 other achievements:
'Legendary Victory' for defeating a Legendary character.
And:
'First of Many Conquests' for conquering your first district.

Good luck

Edit: an addition to the above tactic, if he dodges or parries an attack, roll backwards out of the way straight away and then get back to light attacking, tried it on my sons account with a lower rated dual dagger character and didnt get hit once.

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.