Hey guys as the title says I want some help on how to optimise my fallen aasimar gunslinger. I will be using Matthew mercer's gunslinger and the character is at level 1 right now. I will be using him in the upcoming Waterdeep: Dragon Heist campaign that comes out in September. The stats for my character :rolled + racial stats are shown below.
Cha 11
Dex 17
Str 8
Con 13
Wis 16
Int 12

Any help will be appreciated. 
Thank you

Level 2 choose Archery fighting style so your ranged weapons, which includes guns, get +2 damage.

Level 3 choose Gunslinger.

Choose Deadeye trick shot. You could consider Disarming Shot so that if an ally is next to an enemy and will have their turn before that enemy does you can disarm the enemy and your ally can use it's turn to take the dropped weapon or at least send it further away - this greatly reduces the effectiveness of that enemy for multiple turns or even the rest of combat so it is a good option for teamwork and strategy. Alternatively consider Forceful shot so if an enemy gets in close to you or a spellcaster you can use this to put greater distance, can also be used to manouvre enemies into traps and hazards or off ledges. You can also consider winging shot to make the enemy drop prone - you can use this on an enemy next to your melee-based ally so that ally, if their turn is before the enemy's, gets advantage on all their attacks or you can use this on a flying enemy to possibly bring them back to earth with a thud but be careful with this one as you have disadvantage to attack at range against prone enemies.

Level 4 you have options:

- consider ASI +2 Dex
- consider a +1 dex feat to not only get your modifier up (as you are at 17) but get more benefits. Resilient: Dexterity is a good one for that Dex Saving Throw proficiency - dex and con saves are the most common types of saves due to being targetable by both spells, natural events, traps and hazards.
- consider sharpshooter feat to increase your shooting range, ignore some cover and have option to take penalty to hit to increase damage making it effective when combined with Deadeye trick shot.
- consider the Firearms Specialist feat (turn on playtest material for character) - to be honest sharpshooter is a better feat unless you plan on dual-wielding your guns. If you plan on dual wielding the guns then take this feat.

Of these options the ASI and sharpshooter feats are probably the strongest for consideration unless you really want to dual-wield guns in which case you can opt for the specialist feat (the extra attack is worth the lack of increase to attack bonus).

If you really want to be optimal go Elf or Half Elf instead of aasimar. Their +2 Dex will help considerably and make it easier for choosing a feat at level 4 and you can choose the Elven Accuracy feat which is powerful when combining with Deadeye trick shot. Basically whenever you have advantage on an attack, like you can give yourself with Deadeye, you can take one of the d20s and reroll it. So, you could roll two d20s for the advantage, reroll the lower one, and still get to choose the higher of the d20s you now have to use. It's basically a free Luck point for the attack and so you have greater chance to hit and even increased chance to crit. You're not going to get advantage to attacks that often, being ranged, outside of Deadeye so this will help you really capitalise on it when you do to make up for that. An aasimar offers you no real benefit as a ranged character: no bonus to Dex, their aura abilities are melee only and so on.