Targets of Opportunity are basically mini boss fights found in different locations around the the different areas. There are 10 battles to complete in total (please see below for more details) and you can check to see if you have already completed the target for that area as the Target of Opportunity will be a challenge you can check in the map.
I would highly recommend going for this achievement once you have completed the story so all of the areas are unlocked and the hunts are easy to take down.
Pandora Devil’s Razor 0:00
The Splinterlands 0:33
Cathedral of the Twin Gods 1:16
PrometheaMeridian Metroplex 2:00
Skywell-27 2:20
Lectra City 2:51
Eden-6The Anvil 3:35
Jakobs Estate 4:16
Ambermire 4:42
Nekrotafeyo Tazender Ruins 5:22