Prototype Juggernaut

Prototype Juggernaut is usually on the north side of Trinity Bridge, right in the middle. It's no more difficult than the random Juggernauts that spawn in the area. The only difficulty is that the AFS NPCs in the area might steal your kill, so be sure to out-damage them. 
This target is also needed with the A Bridge Too Far mission and can also be used to get the item required for the Giants Of The Abyss mission.
You also need to kill him as part of the Plateau Targets of Opportunity objective "Kill 5 Major Plateau Bosses".