Overwatch 2

Reinhardt's Barrier Shield ability allows him to hold a large transparent shield that can take up to 2000 damage per full use. It also allows players to shoot through it from your side while not inflicting damage from incoming enemies.

This is most easily unlocked while on offense during a payload objective game. Slowly move forward with your teammates and use your shield against incoming damage. Once the shield is low, find a safe spot to recharge it which will take about 9 seconds. Rinse and repeat with 4-5 shields until the achievement unlocks.

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

What does Barrier Field do?

Barrier Field is bound to Reinhardt's Secondary Fire which is by default.
Reinhardt projects a 1200 health barrier in front of him. His allies are able to shoot through the barrier, but it will absorb 99% of enemy fire. When the barrier is dropped, it will start recharging after 2 seconds with 144 health per second. Recharge will stop if barrier is activated.
The 1% Barrier Field cannot block is Winston's Tesla Cannon, Symmetra's Energy Ball, Moira's Coalescence, and melee attacks, but they will still damage the barrier.

How do I unlock I Am Your Shield?

You must block 7500 damage and you must not die. If you die the damage blocked will be reset. This can (and must) be done with multiple barriers as a full barrier has 1200 hitpoints.
Any damage dealt to the barrier will count towards the achievement, but what are the best ways to block damage?

Chokes

Putting your barrier up in a choke point will cause problems for the enemy team and they will try to destroy it. The best choke points are usually on Hybrid maps which tend to have paths that lead straight towards the first point (as opposed to a flank).

Ultimates

Blocking ultimates is a risk-reward system because some ultimates, such as Pharah's Barrage and Roadhog's Whole Hog, are able to destroy your barrier in seconds and kill you, but a destroyed barrier means you blocked 1200 damage. Some ultimates, such as Soldier: 76's Tactical Visor, will deal some major damage, but will usually not be able to fully destroy a full barrier alone.

Payload

By standing in front of a payload, the barrier are a nuisance to the enemy team and will be top priority. If you are attacking, the payload will also be able to heal small amounts of damage.

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, let them destroy your barrier a few times.

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

To get this achievement you need to play smart and not rush up into the conflict. Reinhardt's Barrier Field ability can block a total of 2000 Damage every charge otherwise it will break. You can lower the Barrier and let it recharge back to 2000 then repeat the process.

This can be boosted if you have a bunch of friends by finding a quiet area on the map then all shooting at the shield, letting it recharge and do it again.

After trying to get this achievement several times with the "play carefully" method I ended up unlocking it very quickly in a strategic location instead.

In my experience I found the attacking team on Kings Row has a great spot right at the beginning to go for this. Right behind the red bus after leaving your spawn is a wall with an opening in it, and the objective beyond it. It just so happens that the opening is about the width of Reinhardt's barrier field. You can hold your shield in this spot, absorb the incoming damage, and simple step to the left side to hide behind the wall for cover when needed. It's important to stay on the left as it will make it harder for enemies to get at an angle to shoot you while you recharge, and if you keep walking to the left, around the corner you'll find a large health pack if needed.

This likely works best when the enemy team has a lot of damage output as it will make you get to 8k absorption much faster, and decrease the chances of your team getting upset that you're not pushing forward. It's also a great opportunity for your team to get ultimate charged up before pushing the objective so it's a win-win.

I managed to get this on the western style map on defense. Walk out of the spawn just a bit and use your shield. Back up into the spawn which also refills your health almost instantly, and let it recharge. Rinse and repeat until the achievement pops.