Grav combos with everything. But with DVa bomb, if you bait it well, EMP is better than grav.
I play a ton of DVa at diamond level. Step one to getting huge DVa ults is to watch all the videos on Level 5 Pidgey’s YouTube channel about ult locations. Then watch them again. Surprise is your friend.
Next, you’re going to want to practice boost control like crazy. Go into a custom game in Chateau and practice all the techs. Dive rocket the apple, boost rocket statue head, do corner peeks and retreats, edge boops, high ground escape, learn the melee cancel and the free boost melee… Research everything you can find technique-wise, and practice it all like crazy. 15-20 minutes every time you get onto the game and you’ll be there in a week or two. This will help your DVa play in general, letting you farm ult more often, keep your mech up for longer, and so get more ults and more ult opportunities. It’ll also help you control mech position on the bombs. Learn how the mech slides on surfaces, and how to angle it around corners or through windows. Just set your ult regen to max and practice chucking bombs.
Last big tip is barrier management. Typically, a Rein or Orisa are going to ruin your bombs, sometimes a Winston will. Usually you don’t encounter 500IQ Winston players until Masters and above and it’s easiest to work around a Winston bubble than anything else. Your basic Winston will jump in, bubble, zap until his jump comes off cooldown, and then jump out. Usually his bubble will break in that time. So you’ve got a (probably damaged) target with no bubble and no jump as he’s on his way out, probably back towards his healers. Follow the Winston jump-out with your bomb, and enjoy the show. If you do that, it’s just down to luck if you get 4 or only 2-3, but you’re getting the Winston for sure (and probably 1 or 2 supports).
It’s usually Rein or Orisa that are going to ruin your bombs. Understand the break levels of the shields. Orisa shield has 900 health (she gets the one you see, and the one on cooldown, which is 8sec), and Rein has 2000. Cracks appear at half barrier health, and heavy cracks at 1/4 (225 Orisa / 500 Rein). If you wait for the shield to fully break, and then launch your bomb, you won’t get anybody. The enemy team will scatter as soon as the shield breaks, and before your bomb even gets there. You have to break the shield right before the bomb goes off, when the tank is forced to hold it or stand behind it, hoping to use the last of its health to save their team.
How do you do that? Micro missiles do 128 damage if you hit them all. Bunny blaster is 100dps with 2 sec worth of ammo. DVa bomb fuse is 3 sec from when you hit the button. Keep travel time of the projectiles in mind. Rule of thumb, you can assume that you can fire the micros at the shield, launch the bomb, then shoot the shield with your pistol to total around 200dmg before the bomb explodes. (Yes, this depends on how far away you are from the barrier.) So, you hold onto rockets until you see the second Orisa barrier (within 8sec) hit the heavy crack stage, then rockets, boost, bomb, pistol. Even from long range, the barrier will break right before the mech detonates. Just aim the bomb anywhere near the barrier location, and what the enemy thought was safe is now insta-death.
You can also fool an Orisa by walking a bomb right to the outside corner of her shield. When she’s deployed the second shield (and can’t simply re-deploy), you walk or boost-cancel to the corner of her shield away from the wall (good Orisas should play near a wall or corner). Plant the bomb there and then just walk away. The blast will cover the outside of the shield, the inside of the shield, and around the corner. Orisa’s only chance at saving herself is to walk through her own shield, directly into your team’s fire (and she’ll get shredded). It probably won’t help you get a 4-piece, but it’s a good ult strategy anyhow.
Rein is actually more difficult to do without comboing, because any Rein worth his salt won’t hold up a shield that’s below 500HP. Either you need a coordinated strategy to actually have a Rein-on-Rein shield war, or you’re going to have to Grav or EMP. You can use the missiles+pistol+team fire if you have a good shield break lineup like Soldier, Hog, Junkrat, etc that can also dump 300+ damage into a barrier within 2sec. In that case, make sure your buddy knows what you’re doing, and that he has a full clip before you launch. Wait for a few seconds after small cracks until the shield has around 600-700 health. Right before a normal Rein will take it down and hide. Rockets, boost bomb towards the Rein, pistol, your teammate magdumps on the shield, and hopefully you’ve timed everything correctly.
Good luck.