In the mission "Cutting the Cord", the Scorpion Fighter, and the Romulans next to it, on the roof adjacent cannot be targeted, not even if you get on the raised portion of the roof that the terminal to drain power from the force field is on, and not even with a sniper rifle - though the Romulans can target you just fine. While on that raised portion I mentioned, I could tab-select the turret on the roof WAY behind the roof with the fighter and Rommies.
I have figured out what is going on with this accolade. As suggested, I went to shooter mode. This is what you need to do IN ADDITION to a few other things (unless you can get your elevation higher; I will explain below; shooter mode allows you to jump AND shoot, RPG does not).
What is happening in this mission is Cryptic has put up "see-thru" walls, or invisible walls to prevent anyone from jumping down from the high areas of the map (leaving the set environment)... The invisible wall facing the Romulans and scorpion fighter is just a hair taller than you can shoot, UNLESS:
You have a Catian (cat) on the team or an engineer places a cover shield on one of the taller structures on the platform you are standing on.
If the catian (cat) jumps while in shooter mode they can get the fighter targeted and killed. I personally think standing on a cover shield is the easier way.
It would be impossible otherwise. You'd have to put the cover shield atop one of the buildings (if you are soloing, you must be an engineer with the relevant kit). You can get there by jumping on one of the small pillar-like objects, then jumping onto the building. Then jump onto the shield.