This achievement isn’t too tough, and can be done. Make sure if you cooperate with others for this achievement that they can do their role on their character well, especially Moira, as you’re team will need healing when the opponents attack you. Try to recognize patterns of attacks at the start of the match.
Speaking of opponents, here’s a small guide for each opponent and how to start the match off
Starting Room
Try to recognize how your opponents enter the room. If they start through the ceiling and go through the left door next, they’ll probably go right and then back. Make sure your Mccree stuns groups of enemy’s and fans/slams the hammer on these large groups. Don’t engage up the stairs too far-you may just die up against too many enemies.
Assassin-Make sure to melee attack her when she grabs someone to stop her attacks. Jump and scatter from her running on the ground. Lastly, watch her on the wall-she may freeze for a second on places.
Heavy Assualt-The heavy assault is by far the toughest special opponent. Stay back! Make sure Genji deflects the barrel guns and avoid charge. Use all ults on him practically! Don’t get caught in his charge-it’s an insta kill on harder difficulties.
Sniper- The sniper is an easy target on easier difficulties. On legendary or expert she might one-shot characters. You can have Mccree high noon her or have Reaper teleport for an easy kill.
The rest is smooth sailing- bounce and jump heavy hitters and group towards the drop ship immediately at the end, don’t get distracted.
Happy hunting!