Q > W > E > R Skilling Order. Testing will needed to be done simply for the possibility that Deadly Volley may Apply on Hit effects because of the skills wording but for now assume it doesnt. If it Does Skill order would be W > E > Q > R
1A. Sharpshooter Considering how far its fallen and how underwhelming it can be in comparison to a lot of the other proficiencies, Sharpshooter will most likely be the Go-To Proficiency to rush. However its possible that it may not give you enough meaningful damage because the nerfs the proficiency took really did gut it quite a bit. Despite this, Sharpshooter has almost everything Itarri wants including that much desired MS increase.
1B. Void Damage Void Damage simply is one of the hardest hitting proficiency and for anyone looking to carry it is currently the Go-To especially with the current pricing System. If Deadly Volley DOES apply on Hit effects, it could make this Proficiency go from strong to god-tier on Itarri, but even without it, how trigger happy Itarri is and how beneficial it is to be constantly weaving in attacks make this a very strong Proficiency as She isnt too reliant on her abilities to deal damage so building Raw weapon damage wont be as Optimal.
2. Carry Damage If you've gone the sharpshooter route then this Proficiency really will just make you shred through the opposition, pretty much gutting any armour the opponents get. Whilst the high %Pen is the main attraction and is strangely strong to get early in Supernova, The High amounts of bonus Weapon damage will also give your attacks that bit of extra punch.
3. Carry Bonus I am slightly iffy on this one but on paper it has a lot of potential to synergise with Itarris' Ultimate Passive, made even more potent with the strong amount of Penetration. Testing will need to be done to see how much potential it truly has, but it couldnt definitely be a contender.
4/5. Armour Shattering and Carry Burst These two are sort of interchangeable in there positions thats why ive put them together. Many people seem to dismiss Armour shattering as a Proficiency which surprises me because it has A LOT of raw power behind it. The stats are definitely a wee bit confusing (Crit Chance with HP on an Armour reduction proficiency? Who is that aimed for?....) But even with a weird aim behind the proficiency it currently has a lot of power in the shred and ive found myself picking up Armour shattering often as the second proficiency on most bruisers just because its such a good mix between defense and Offense. To compare it, its almost like the current iteration of Black Cleaver on League of Legends. Its weird, its Confusing, But my god is it so good.
Carry Burst sort of needs no introduction, It gives you more damage, You hit harder and combined with armour shattering dayum will you be critting often and hard.
Some are obviously interchangeable as games will vary and you DO need to mix up builds and playstyle, but Itarri will need to be in the fray a lot and relies heavily upon being a slippery little nuisance to cause as much disruption as possible. I'd recommend playing her in the Mid lane, and using her Evasiveness and Mobility to heavily pressure the sidelanes.
Whilst you're in lane however you want to be constantly trying to tag your opponent with restraining shots and constantly chipping away at their health. Because of her playstyle Itarri seems to be a character that if you stick around even on say 60-70% HP, She can and will hunt you down with her vast mobility and plethora of spells.
A lot of this is depending on Various interactions and information and may be subject to change. Crucial info such as Internal Cooldowns on the CD Reduction of Blitzvault and Internal Cooldowns on the Spectres Passive will heavily influence Itarris Strength.
Ill see you on the battlefield Lads.