Tier 1: Barbed Shothelp alot with clearing creeps and camp while others are not really effective.
Tier 2: Envenom give you a huge burst damage
Tier 3:Taking Unstable Poison work best with Barbed Shot in tier 1 since it will clear the creep wave in a blink of an eye and also deal huge amount of area damage, especially in Spider Queen map, The spider Queen will disappear in a few second, and also help in teamfight when initiate with a lot of creeps.
Tier 4: Wailing Arrow2.5 seconds silent is a game changing skill when you land it nicely and do a lot of damage as well.Since Possession don't do anything much, controlling a normal creep for 4s is nice but a siege with 20s cooldown is not and lv20 can control Mercenaries with 20s cooldown is way too long and they are not even that strong to stay pushing.
Tier 5: You can actually chose between Spell Shield and Splinter Shot, if they got a lot of burst magic damage, chose Spell Shield to stay longer in teamfight, if not, chose Splinter Shot, it give better wave clear and also give a lot of damage, and work well with tier 1 and 3
Tier 6: Cold Embrace give you a lot of dmg to heroes and pretty much everything so it is a must for me which gonna give high burst of dmg.
Tier 7: You can take Deafening Blast if you are confident that it gonna hit. Or you can take Bold of the Storm for more mobility to position in teamfight

In lane, just spam Q since it cost no mana on creep and talent lv1 give you the potential to clear them fast.
Lv4 after taking envenom with a lot of damage you can go solo gank to help other lanes.
In teamfight, the combo should be E>R>E>R>W>QQQQQQ and then spam WQ, this is an agressive style and need some practice to get timing right since you should blow your ult when you land in the middle of the teamfight and start spamming. Or you can just play safe by wait for team to initiate and chose a good spot to land your ult and start spamming and keep E to position yourself or run away.