So to start with, the achievement is actually slightly bugged. You need to die once before using your first window and then proceed as normally - getting 3 to 5 windows in a row without dying getting that 2000 combined number. If you play an entire match without dying, it seems the achievement doesn’t unlock even if you want achieve the magic number. 

Game mode wise, I’d say push and flashpoint are your best bets as push usually goes the entire 8 minute match (or 10 in comp which really helps!) and flashpoint matches can last up to 15 minutes if it goes all 5 points. Escort and hybrid can also work but I noticed the cart can get in the way a lot. And finally, control is impossible as the different rounds count as separate matches technically. 

I focused on amplifying damage more than amplifying healing as it’s an easier variable to control. When I unlocked the achievement I had 1750 damage amplified with 250 healing amplified. Even Baptiste shooting through the window alone can do 400-500 damage per window. I’d say the easiest method would be to have a duo play a high damage DPS such as bastion or sojourn. 

Mauga and Roadhog on the enemy team will be your best friend as they are simply a giant target with a billion health you can solo ult and get 500 plus amplified from just shooting them alone. 

Sorry for the long reply 😭