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DPS
There is a popular belief that having a sniper is a hard counter to Mercy. We'd agree at lower ranks or skill levels, but Mercy players with strong movement can be incredibly difficult to hit. For this reason, we're going to err on the side of caution and recommend DPS heroes that are not as aim-reliant.
Take for instance heroes like Cassidy and Ashe. While he is very aim-reliant, Cassidy does still have access to his Magnetic Grenade. Playing on either the frontline as a tank buster or a close flank could be the gift you're looking for.
This positioning naturally threatens the Mercy and allows for heavy pressure to keep her moving. And if she's moving, it's harder for Mercy to keep line of sight on her pocket and/or she can't be resurrecting.
And God forbid you get in close enough for a sticky nade? Cassidy is a solid counter if the opposing Mercy is giving you trouble.
In a similar vein, Ashe provides her own counters. Dynamite is the golden ticket here. Not only does this naturally counter Mercy due to her proximity to her teammates but this can also be a tool to fight her movement as well.
Let's say you've playing opposite a cracked Mercy with movement you've never seen before. Hitting that Mercy is going to be next to impossible. However, having access to an AoE tool like Dynamite could be the difference maker for your team in clipping Mercy's wings.
If you're looking for a more creative take, try looking at Mei when aiming to counter Mercy. Think about who Mercy is often paired with, and no we don't mean Pharah.
Sojourn, Ashe, Widowmaker, even Cassidy or a Hanzo at times--if we shut out their line of sight, we buy our team time.