For those trying to do this legit, I suggest not even bothering on game modes other than escort and hybrid. The other modes are not long enough without getting extremely lucky ults. Some enemy heros also make this very difficult - if the enemy has a doomfist, widow, genji, etc, it can be very hard to stay alive the entire game.
I unlocked this on Rialto, and it took 5 uses of my ult to get it. Being on defense is a little easier, because you can purposely throw the game a bit to make the game last longer. Rialto is a decent map for this as there are not too many flank routes with the exception of the very beginning of the map.
Once I had my ult, if I could see 2 enemy tanks or at least 2 or 3 enemies, I ulted and started firing. Fortunately, even alone as Baptiste you can often do 400 to 500 damage. Orisa/Reinheart/Zarya are all great team mates to have to maximize damage through the window.
I do not suggest trying to get this by healing. I would sometimes throw some heals through the wall if allies were very low, but I found that you will not amp nearly as much going only via healing.
Save your abilities to save yourself. Good luck!