Evolution II : Perk 1: Marksman's Hand: -40% Recoil.
-> no recoil with steady hand.
Evolution III : Perk 1: Lethal Rearmament: On Headshot: +30% Reload Speed for 12 seconds. Stacks up to 3 times.
-> you have to headshot to proc incarnon mode, so free reload speed.
Evolution IV : Perk 3: Elemental Excess: +20% Status Chance. -10% Critical Chance.
-> to help the evolution V to proc (flat modificator, calculate after the mod).
Evolution V : Perk 1: Devouring Attrition: 50% chance to deal 2000% damage on non-critical hits.
-> it's MULTUPLICATIVE.

Why not galvanized shot ? don't work on AoE.
You can replace gunslinger by anemic agility (+18% fire rate, -15% dmg), it's a matter of taste.
Why corrosive and not viral ? slash disapear in incarnon mode, most tanky unit are resistante/immune to viral and have a lot of armor, so corrosive is the way. (you can switch if you want, or have nourish on your frame)

for the arcane :

• secondary merciless : dmg bonus is easy to sustain, just kill, have a 30% recharge bonus (with the 3 stack of evolution III, you have +120% reload speed, not bad)
• secondary deadhead : you have to headshot to dmg bonus, you have a +30% dmg multiplicator on headshot (good for unit with the trash mecanic of adptative armor) and a useless -50% recoil (we are already at -100% with steady hand and evolution II)
• cascadia flare : on heat status, +12% dmg for 10s, stackable to 480% (on this build, every status have a 50% of being heat), so +480%dmg vs +360 for merciless/deadhead, better, especially on boss whith no grub to kill BUT all stack desapear at once when expired BUT the heat proc don't need to come from the [Laetum]