Overwatch 2

What i did was wait for an enemy like roadhog or reaper to get in their ult because it last a while, then just simply throw your flashbang on them and if you see them dazed from it, then you know you got it.

McCree's Flashbang ability allows him to throw a flashbang grenade that stuns enemies at close range.

You'll need to wait for an enemy to build their ultimate and hit them with a flashbang as they do it. It works against many common enemies including Genji, McCree, Reaper, Soldier 76 and Roadhog, among others. It will not work against Bastion and "single use" ultimates like SYmmetra, Tracer, Widowmaker, and Zarya among others.

If you'd like to learn more about tracer and this achievement, you can view this video:

What does Magnetic Grenade do?

Magnetic Grenade is bound to Cassidy's Ability 2 which is by default.
Magnetic Grenade homes in on enemies close to the reticle. If no target is found, it will travel 10 meters. If it chases a target, it can travel up to 13 meters. It can deal up to 131 damage (1 impact, 65 explosion, 65 directly to target) to the stuck target and 65 damage to non-stuck enemies from its explosion.

How do I unlock Gotcha?

Gotcha requires you to get 2 killing blows (not just assists) so it's a bit tough. While this is easier said than done, you can get a shot onto a 200 health hero, stick the grenade onto them, and get two body shot/one head shot onto another 200 health enemy that will take splash damage.
Alternatively, some ultimates that stun, such as Reinhardt's Earthshatter and Mei's Blizzard, or Zarya's Graviton Surge can make this achievement a bit simpler.

How do I boost this achievement?

The Lockdown Method

This is the most common method to boost Overwatch that requires 10 people.

1. Get a group of 10 people.
2. Get into two groups of 5
3. Disable cross play
4. Queue for Quick Play Open Queue at the same time and hope it matches you guys up
5. Once you match up, get two enemies' health to a little over 65 and under 65 and have them stand together. Stick the grenade on the one over 65 and you should get the final blow on both.

Remember to keep the objective contested unless it's a Control map which will not need contesting unless someone captured it.

A couple other ways to get this.

--You can absolutely get this on Baston. HOWEVER, you may need to hit him during his transform, not before or after.

--A great time to throw your flashbang is at the end of a round. Many people will use their Ultimates at the end of a round, since it'll disappear anyway. Throwing your flashbang can catch these people.

--You can also unlock this achievement while playing as Genjii. Reflect a flashbang into a ultimating enemy to unlock the achievement. Enemy MeCrees will throw before starting deadeye. Be ready to reflect, and you can unlock this.