I’ve no idea what’s involved in the achievement version but the boss in Rogue Agent was the most reasonable of the three missions at e9, since he had about a third of the health that bosses did in Missile Crisis and Car Park of Shenanigans. I quit Missile Crisis halfway through and did it on e4 instead - it was no more difficult on e9, just much more time consuming.
Also, I’d like to suggest making these missions playable in groups. It’s a multiplayer game, you should be getting rid of the “solo instance” stuff, not adding more. Seriously, all the missions that casual players find the most frustrating turn out to be the ones they can’t be helped through and that’s really annoying.