Overwatch 2

Pharah's Barrage ultimate ability allows her to launch a barrage of rockets damaging enemies below. Although the ultimate ability is not necessary for this achievement, it will help tremendously.

We will need to kill 4 enemies without touching the ground. A few things to keep in mind are that you can Jump Jet twice without touching the ground if you start your first jump on a high section of the map. Also, the barrage should grab a few kills, but feel free to mop up the last 1-2 kills with your regular rocket or concussive blast before landing.

I recommend going for this while defending the payload and waiting for a group of enemies that are trying to move it. It is best to pair up with a Mei or Zarya that can group together defenseless enemies for you allowing you to finish them off quickly.

If you'd like to learn more about Pharah, you can view this video. The achievement is described and shown at 9:40:

Edit: This has gotten a lot easier since they lowered her fuel consumption, as you can now essentially stay in the air forever.

Unlike most hero multi-kill achievements, Pharah's doesn't NECESSARILY have to be done with her Ultimate, but obviously it helps. Pharah is built around staying in the air and being hard to hit.

In order to maximize air time, your best bet it to start a combo with your Ult and get as many kills as possible. As this is an objective based game, you're bound to end up at a point where people will be clustered on an objective.
Once your Ult has been used (and if you're still alive), keep tapping your fuel to stay in the air as long as you can.
Once your fuel is out, use your Jump Jet to jump back into the air before hitting the ground, and you should have some fuel by the time you're back in the air. This will maximize your air time.

All the Capture maps are fairly good for this achievement since they all have at least 1 Capture Point that has a cliff Pharah can knock people into with Concussive Blast. Ilios has some cliffs at all 3 points, though the one in the ruins is only going to catch flankers. Nepal has the big indoor area. Lijiang tower has cliffs at all 3 though most people don't go outside at the major indoor part. Nothing beats that feeling of knocking multiple people off a cliff.

Escort (defense) it's going to be slightly harder since her ultimate is a bit more over time than some heroes, and if they're running around the payload enough they could heal a good bit while dodging you.

Now it's entirely possibly to get all 4 kills with Barrage but this is just to get you as many opportunities for kills as possible. Even if you are in a tight space, always try jumping before your Barrage, as it could easily turn into a Quad Kill. People don't generally expect a tight quarters Barrage.

While her Barrage does damage over a fairly wide area, it can help to be closer to the ground to make sure you focus a target down better. If you just shoot over a wide area, a Lucio could easily keep a team healed. Focus down any healers, then go for the other squishy characters. In my video though it does seem to kill a lot faster than I had anticipated from long range.
Pharah's Rockets can 2 hit most heroes if you can get a direct hit, though the splash damage if you miss is pretty terrible.

As with most Multi-Kill achievements, teaming up with a Zarya or Mei Ultimate is a great benefit.

Edit: This has gotten a lot easier since they lowered her fuel consumption, as you can now essentially stay in the air forever.

Unlike most hero multi-kill achievements, Pharah's doesn't NECESSARILY have to be done with her Ultimate, but obviously it helps. Pharah is built around staying in the air and being hard to hit.

In order to maximize air time, your best bet it to start a combo with your Ult and get as many kills as possible. As this is an objective based game, you're bound to end up at a point where people will be clustered on an objective.
Once your Ult has been used (and if you're still alive), keep tapping your fuel to stay in the air as long as you can.
Once your fuel is out, use your Jump Jet to jump back into the air before hitting the ground, and you should have some fuel by the time you're back in the air. This will maximize your air time.

All the Capture maps are fairly good for this achievement since they all have at least 1 Capture Point that has a cliff Pharah can knock people into with Concussive Blast. Ilios has some cliffs at all 3 points, though the one in the ruins is only going to catch flankers. Nepal has the big indoor area. Lijiang tower has cliffs at all 3 though most people don't go outside at the major indoor part. Nothing beats that feeling of knocking multiple people off a cliff.

Escort (defense) it's going to be slightly harder since her ultimate is a bit more over time than some heroes, and if they're running around the payload enough they could heal a good bit while dodging you.

Now it's entirely possibly to get all 4 kills with Barrage but this is just to get you as many opportunities for kills as possible. Even if you are in a tight space, always try jumping before your Barrage, as it could easily turn into a Quad Kill. People don't generally expect a tight quarters Barrage.

While her Barrage does damage over a fairly wide area, it can help to be closer to the ground to make sure you focus a target down better. If you just shoot over a wide area, a Lucio could easily keep a team healed. Focus down any healers, then go for the other squishy characters. In my video though it does seem to kill a lot faster than I had anticipated from long range.
Pharah's Rockets can 2 hit most heroes if you can get a direct hit, though the splash damage if you miss is pretty terrible.

As with most Multi-Kill achievements, teaming up with a Zarya or Mei Ultimate is a great benefit.

Before I start I want to stress that the 4 kills do NOT only have to be from the ult. You can do combination of regular kills ult kills and concussive blast kills. Just be sure if you are using your ult don't be 2 feet from the ground so if you don't get all 4 kills you can rocket into the air and get kills with regular rockets.

As long as you kill 4 players without touching the ground, the achievement will pop.

I earned this by killing 2 players with normal rockets, using the jetpack boost (I forgot the name of it) and then using the ultimate to kill 3 more players, killing the 4 needed players in the process.

I haven't tried using the concussive blast to knock a player to their death for this achievement yet, but it should count towards the 4 players.