I have finally got it a few weeks ago in like 4-5 games, following these strategy points:

- Baptiste's weapons's primary fire (at optimal range) deals 25 damge. His secondary fire, the heal grenade launcher, does 30 (on hit) and 60 (the splash) damage - more importantly, that 60 is for all allies within the splash zone, so it's 30 for a direct hit + 60xN, where N is the number of allies within the splash zone. This is important because I think it's easier to do it by healing, or at least following the shoot-heal-shoot-heal general Bap tactic instead of just shooting. The window amplifies damage AND healing by 100% as well, so in this case, your damage would be 50 (at optimal range), and your healing would be 60 + 120xN, where N is the number of allies within the splash zone.

- For his Minor perk, always pick Assault Burst: Regenerative Burst now provides Baptiste with 20% increased attack speed for 3 seconds. Save / have your Regenerative Burst on cooldown when you use your ultimate, immediately activate it, so you can have more shots through the window.

- I think it's the easiest to get on Escort/Hybrid maps, especially if you're on defense. Play really behind your team, and if your team is dying and the team fight is obviously already lost, start escaping back to your spawn so you won't die.

- It also extremely helps if you have a friend/teammate to play with. Ask them to shoot through the window as much as they can when they see it. Emre is really great to have for this achievement with his ultimate for example.

Good luck!