The Ruinous Road

It can be found on top of the overpass (560,400). You can get to there from climbing the debris at around (465,400).

It appears to have a respawn time of around 3 hours. Its attacks aren’t particularly hard-hitting, however, it does have numerous AoE ground attacks including several circles and a cone attack. Be sure to avoid those. Also, it’s recommended that you have acquired the shield controllers before attempting to kill this monster alone.