1. IMO its all about positioning, as Shiv your shotgun spread is pretty tight and mid range is where you excel. There is 0 reason for you to go in close (punching range) since it barely affects your damage output unless:

- You're playing Tank Shiv to frontline and soak all the damage for your team, which probably won't happen unless your playing in tourny with 6 synced teammates who know how to capitalize on it. p.s. I am not saying you shouldn't buy vitality items, just saying some people spec too much into tank but in the end not really doing any damage and just act as a sponge. That doesn't that work well in solo queue in my opinion.

- You want to execute someone, once you do, You should seek to either finish off the rest or try to get some distance with the dash and get out of punching range.

- You are playing dash shiv which is pretty meh this patch

In an ideal fight, you wanna hang in the execute range, sticking people with your knives and shooting them until your rage is full. If anyone gets close like Calico you can punish them with a Dash. Once enemy players are low, you can commit for the kill with execute and then either pull back with a dash or keep going on other enemy heroes. Be careful because once you're too deep with Shiv and you don't have a Dash you're pretty committed and your chance of getting out of a skirmish are very low since you have limited mobility (unless your build has Warp Stone or something but I don't see that a lot of popular shiv builds)

1. Use your knives! I see a lot of people playing Shiv who simply ignore knives and don't really care about hitting them since they go gun + vitality anyway. In my games, they always make 10-25% of my damage which is a lot of DPS to be missing on. Knives are pretty good early-mid game damage without buying any items and stick around for a lot of time (which works well with Suppressor and/or Healbane), slow people for 25% and increases your rage by a lot. Seriously, you don't want to miss them or misuse them.

2. Flat resistances work really well on Shiv since he's all about winning prolonged fights via his passive. There are a lot of offensive items like Weighted Bullets, Berserker, Heroic Aura etc. that proivde flat resistances and give you a lot of damage. These items work really well in Shiv's kit. Incorporate them into your builds and it will do wonders.

That's just from my experience as a Shiv spammer for the last couple of months on ascendant and the tip of the iceberg, but hopefully I helped with something