Considering his ptr rework coming soon take this with a grain of salt as it may be obsolete then but for now he has a few ways of working well.
1 counters..he has many and just like a pharah if no one is shooting you out of the sky you can do work, so know when your time is up on him as he can be the hero that carries or throws a game.
2 Target priority. Knowing what the most vulnerable enemies are ie out of position (and also in your positional area), low hp targets/200hp heroes that can be taken down with a combo (more will fall if you get lucky and or have team support to back you up, like a winston diving). I will add into this the fact he is VERY GOOD at backline peeling for your supports or any other team mates with a flanker on them as you can jump back there fast and put pressure on them to keep your backline secure, so sometimes your target priority turns to defending your team rather then getting picks up front.
3. Rotation - Doomfist has many combos that he can do depending on the situation and watching some Chipsa on twitch will show you how each can be pulled off as some are alot easier then others. and theese are just how each ability can be used in conjunction with each other.
SS - Start the fight from a high point if possible and drop in with a Seismic slam (more dmg from height) and at full power can get most 200 hp heroes under half health as you land. I like to do the genji 180 trick on the leap to 180 around to the back of them (when you can, direction is only negated by obstructions or your next move ie hit them into a wall so position that to get a good angle on the target as well as getting more then 1 in your area of dmg.
Uppercut is a dangerous ability that leaves you in the air so keep that in mind if they have a hitscan to melt you up there. learn to use your gun here, both in the same animation as the uppercut so you shoot,uc, then shoot a few more times if you can secure the air kill if not retreat with a charge fist away. If you can get a kill from the uppercut think quik for your next move to either charge away or try get another kill. UC has a 2nd use that is very handy as it interrupts alot of abilities/ultimate's when you think you can drop in and get that mcree for example (all 3 of his abilities work for interupts depending on the ability being used.
Rocket punch is a great tool for mobility and for the wall hit combo that with great timing can be used for both. with mobility hitting a small object infront of you gives you a jump up (like a moira jump) so that can be used for vertical movement as well as getting over a building with speed.
my basic rotation is SS, shoot, UC, shoot x?, then charged away/next kill if i have team support. Drop from high with a full charge, 1 shot some one then SS to the next target and then UC and then a charge if it is up.
Quick tip on rocket punch learn how far just a tap will send you and its damage value as you may not need to fully charge the punch to escape or get the wall kill. His ult can save your life from tight spots and never be afraid to use it to get out alive as it charges very quickly.
