If you play on PC, turn on the healing assist (Setting > Battle > Healing Assist) and your job is easier because you only press Tab button to switch between low health teammates. Always spam 2nd and 3rd skill and in talent tree get your Flourish node on max lvl so whenever you cast soul link (4th skill) you can cover all over your teammates because you have full datura flowers.
You can start Shadow Blade and then summon Spectral. Use Soultrap to gain soul energy and you can start to cast Soul Elegy and Bone Spirit, but pay attention to your soul energy when your Spectral is about to run out, keep minimum of 2-3 soul energy. Can also use Soul Arrow for you filler.
Just keep your soul energy at least 3 if you have you Soultrap skill on cooldown so in the next Soultrap you have enough energy to summon Spectral.. Other than that just spam Bone Spirit (the 3rd skill one)
I consistently do around 12-1300 on st and aoe depending on how many mobs there I have hit as high as 3700 burst. Make sure you have uptime on your shadow burst and keep your spectral up at all times. Always start the fight with a shadow blade and summon spectral followed by a soul trap, into bone spirit spam, use soul arrow as a filler when you need to buy yourself some time for a gcd. If you are almost in need of refreshing your spectral, keep an eye on your shadow blade cd, you can exhaust your souls with bone spirit and use shadow blade like you do in the opener to generate a quick 5 souls for the spectral. Also, casting soul elegy when it procs (grants a free cast and gives you a soul) it’s just free damage. Just make sure to be careful when using soul burst so that you don’t eat all of your souls if you need to bring a spectral out, unless you have your inscribed spirit buff ready and can generate souls quickly.