Considering that this has been adjusted for Overwatch 2, I’d like to submit my guide. I got this as soon as Baptiste debuted, but helped a friend get it last night.
Window of Opportunity now states 2,000 in healing of damage amplified. That’s down 500, but there are two less players in a given game. The key is to focus on the healing. You have much more control over amplifying heals than you do generating meaningful damage. It is too easy for an enemy tank to absorb or block that damage, or even just dip out of view. You don’t need a full team, but I recommend having two friends play tank to well… tank damage to receive healing during your ultimates. Don’t bother attempting this on control maps as the counter resets with each round.
My video guide:
Window of Opportunity - Overwatch 2’s RAREST Achievement Made Easy