Core Stats
STR 5
DEX 5
AGI 10
CON 3
PER 11
WILL 3
INT 6

5 points leftover, options will be discussed in the Feats and Specializations sections

Guns: max every level
Throwing: 80 effective for 4 tiles range or 123 effective for 10 tiles range at max precision

Stealth: 100 base points could be more, could be less, it's the first skill I would take points from, it really depends on you and how many more skills you want to invest in but all the benefits of stealth are too good to pass
Hacking: 105 effective, pass every worthwhile check, Gear section for more info
Lockpicking: 98 effective, same as above

Some notes about crafting to understand my choices:
1) The crafting benches bonus comes after the values shown, and will put the intended max in ( ).
2) The max values also have some items in mind, check Gear section
3) I calculated them with 160 quality components at max in mind, grinding for higher is too time consuming but if you do find some just use hypercerebrix to get higher values

Mechanics: 102 (128) effective to craft your sweet sweet boomstick of doom, also known as shotgun
Electronics: 136 (167) effective to craft an energy shield with 2 modulators
Chemistry: 60 (69) effective to craft Mk IV grenades, MK V aren't worth all the investment
Biology: this depends if you want to craft supersoldier drug and regen mixture in DC so 130 or 115 effective, without benches, but 90 effective is good enough for super health hypos. It could be even less depending on what drugs you want to use or if you want to use the benches. Get at least enough for Focus Stims if nothing else.
Tailoring: 134 (154) effective to craft leather armor

Psychokinesis: 45 base for Electrokinesis and Electrokinetic Imprint
Temporal Manipulation: 70 base for LTI, Contraction and Statis

Persuasion: you could potentially never use it and be fine but I find the RP value, as little as it is, and being able to skip things or make some thing easier too usefull to pass, check the wiki for your preferred threshold. I personally find a certain encounter in DC where you can just make an enemy stop and skip a fight too funny not to see it everytime. For that, 90 effective while bringing boots and leather armor made from heartbreaker, or 95 effective with a White Dude is enough.
Mercantile: 105 effective is almost mandatory for any build that wants to craft

Core
Sprint: gotta go fast, position is king with shotguns, also helps with blitz, check How To Play section
Sixth Shell: the damage is simply amazing, take it as early as you can
Premeditation: free stun with both psychokinesis abilities, also helps with psi cost
Leading Shot: those pesky evasive people thought they were safe from my shells, they were wrong
Kneecap Shot: melee characters are now walking sacks of sadness, also procs opportunist
Blitz: 20 ap is another shot, which is very good
Barrel Stare: great damage boost. Now you might be thinking: "Both leading shot and barrel stare? Are you dumb?" and the answer would be yes, but you can always choose to stand close to someone and get the boost if they aren't evasive, especially those heavy armor guys, and if you aren't , sure all those movement points help. Also shots from stealth ignore evasion, since they're suprised, while stunned/incapacitated targets have theirs disabled. Walking to the most evasive pipeworker in stealth and blasting him to kingdom come is always a treat
Hit and Run: as I said earlier, position is king and being able to waltz through a map while also killing everything that moves is great, also helps with Blitz, check How To Play section
Psycho-Temporal Acceleration: Contraction is now a bonus shot, need I say more?
Fragmented Chaos: this build is all about crit, so those crits hurting more is good
Pellet Mayhem: more crit is always good
Increased Perception: you start with 9 PER which is potentially enough but you need 11 for the next feat
Scrutinous: crit good, lots of crit gooder

Optional
Nimble: no armor penalty with the intended endgame armor so you can run even farther with all those mp points, also stealth
Opportunist: damage boosts are always nice, it's also easy to proc with kneecap shot, your psi stuns, flashbangs, stingball grenades or the almighty taser
Blindsiding: another damage boost, very good. Less good in extended fights
Recklessness: more crit good, but it's very little and there are much better feats to take before this, maybe at level 30 to max those hilarious people erasing shots

Optional Stats
Grenadier: 6 DEX very much recommended but still optional, flashbangs and incendiary grenades are great, using them even more often is even better
Escape Artist: 7 DEX and 30 dodge, helps with the natives, mutaded dogs and anyone that likes to use nets. Nothing that a good shell to face before they even realize you're there can't fix
Expert Sprint: 13 AGI, 10 ap means an extra shot but it comes so late that you might not even need it
Skinner/Gun Nut: 7 INT I don't like them, the bonuses are so minor I don't think they're worthwhile but they're here if you don't like anything else

If you don't want any of these you could just put all the excess Stat points in PER and go guns blazing without a care in the world about yourself or your F9 key, or put them in CON so you don't die from a sneeze in your general direction

Sixth Shell: 5 points as soon as you get them, there's never too much damage
Hit and Run: 2 points, can put them at any time, they allow a simple strategy with Contraction, check How to Play section
Premeditation: 2 in cooldown so you can use it more in extended fights, which is almost always if you use Statis
And now for a little tangent that ia honestly kinda dumb and overcomplicates things:
Let's say that for some reason you want to use Electrokinesis, Contraction and Statis in the same turn, which will probably be most fights, without using a Psi Booster, which is dumb but hear me out, you would need, if my calculations are correct:
6 INT ,so you can get Philosophy 2 with Hypercerebrix, and 4 points in Premeditation psi cost for exactly 110 psi points
OR
7 INT, same as above but for Philosophy 3, and only 2 points in Premeditation psi cost
OR do neither like a normal person, read not me, and use a doctor's pouch
Leading Shot: I honestly don't know how much of a difference it makes since I maxed it second while playing a beta version of this build, put the leftovers here or anywhere else you like

Small note about shells: 20p against anything that doesn't have armor and 6p against armor. 12p are meh, 3p have their uses against extremely tanky targets like crabs, strongmen and nagas but are heavy and merchants rarely have them, 6p is still enough even against those.

Combat
Shotgun: Vindicator, Short Barrel, Forward Grip. 20 AP shots while also maxing damage, will melt people
Head Slot: Seeker Googles, you absolutely love more crit and this gives a lot. You could aldo make them night vision, which I recommend, for better accuracy in darker areas
Armor Slot: Infused Rathound leather, Black Cloth, Soft Foam Padding leather armor. Even more crit with a good chunk of stealth, very nice
Boots Slot: Infused Pig(look up Fatso for highest quality) leather, Boot Spring, any padding(I used Galvanic Foam Padding which when combined with a certain armor from the DLC makes you immune to electrical damage) leather boots. Extra carry capacity and huge MP bonus is great for general use but remember the stealth malus, I had so many goofs because I kept forgetting them
Belt: the saner options would be a doctor's pouch but a shell strap belt is still an option since you shoot so many times each turn
Energy Shield: There are so many combinations and since they're very light I would definitely bring more than one. High/High efficient against bullets, Low/Low efficient against melee and crossbows(Another option would be to use a low quality, 50 or less, Circular Wave Amplifier to better resist crossbows, as those are the bane of any glass cannon), or any combination of the two plus Medium
Combat Utilities:All the grenades have their uses, tasers are simply godlike(lowest quality Electroshock Generator, usually from Old Jonas, will get you more uses and still stun, but remember that it needs to di damage in order to stun so be careful about people with higher electricity resist).

Crafting and Subterfuge
Stealth Gear: all the usual stuff from black cloth, balaclava, tabi boots and a cloaking device
Hacking and Lockpicking: Huxkey, Jackknife and Lemurian Engineer Suit to reach those 130 thresholds
Biology: Bioinvestigative Belt to free 15 points and having better health hypos, but it's entirely optional and you could choose any of the other Institute belts or none at all
Mercantile: The new Large Waist Pack adds 15 points to use elsewhere
Crafting: Once again the Lemurian Engineer Suit for mechanics and electronics
There are a couple more items that increase those skills but I didn't want to juggle so many and tried keeping it at minimun

Your ideal turn would be, get close to someone and shoot them point blank from stealth, MP from Hit and Run, Contraction, blitz and adrenaline for more shots, Premeditation Electrokinesis the most annoying enemy before or after your first shot and just unload on anything that moves. Sprint at any point, after blitz, to reposition if you can hit multiple enemies, if you have Expert Sprint the same strategy applies with the only difference being 1 more shot. Be careful about AP if there are a lot of enemies so you can reload or grenade then Statis, unless you didn't already use Premed. After Statis is over you can clean up.

Maximizing Blitz:
Contraction + Sprint will get you 60 MP, you will probably do this a lot, but as soon as you get 2 spec points in Hit and Run you can now kill an enemy to get 31 MP, then use Contraction and have 60 MP while also having Sprint to reposition if needed

https://underrail.info/build/?HgUGCgMMAwfCoHEAAAAAwqBMTQAAUm0qV2sALQBGXgA6OSTCsBYrKsKWHkfCjxkmwofCmMKQwrXCpDHCneKgigLiorQC4qK1AuKzrATit7AF378

Level 1:
https://underrail.info/build/?AQUGCAMIAwcPDwAAAAAPDw8AAAAAAAAAAA8AAA8ADyQ5378
Very basic, points in all the skills you use that early. I put points in Psychokinesis early so that at level 4 you can get Electrokinesis.

Level 4:
https://underrail.info/build/?BAUGCQMIAwceHgAAAAAeHh4AAAAAAAAAAB4AAB4AHiQ5wrAWK9-_
Just maxing what you already have, Electrokinesis is now available so use it whenever possible

Level 8:
https://underrail.info/build/?CAUGCgMIAwcyMgAAAAAyMjIAAAAAAAAAAC0ABTIAMiQ5wrAWKyrClt-_
Still maxing, also started putting points in Temporal Manipulation for later

Level 12:
https://underrail.info/build/?DAUGCgMJAwdGRgAAAAAyRkYAAAIAAAAAAC0AN0YAOiQ5wrAWKyrClh5H378
Mercantile reached what we want so we can use the points to get 55 Temporal Manipulation and start using Contraction, you get really lucky and find Stasis early you can put 70 points and use that aswell

Level 16:
https://underrail.info/build/?EAUGCgMKAwdaRgAAAAA8TE0AAEQAEA8AAC0AN1oAOiQ5wrAWKyrClh5Hwo8Z4rewAd-_
Stealth is fine as it is, you'll have a lot of bonus with the right gear, also started putting points in crafting which you can do earlier to have an easier time with repair kits, biology and chemistry for gas grenades to cheese The Beast fight. Same thing as above with stasis.

Level 20:
https://underrail.info/build/?FAUGCgMLAwduRgAAAAA8TE0AAFItKjMAAC0ARl4AOiQ5wrAWKyrClh5Hwo8ZJsKH4rewBd-_
Now crafting really starts to open up. I recommend maxing Mechanics first so that you can craft your endgame shotties but if you're unlucky you can always delay it and put the points in electronics for better energy shields. With the benches you can start crafting Napalm incendiary grenades, 70 biology for Focus Stim and Hypercerebrix.

Level 24:
https://underrail.info/build/?GAUGCgMMAwfCgkYAAAAARkxNAABSRCozawAtAEZeADokOcKwFisqwpYeR8KPGSbCh8KYwpDioIoC4qK0AuK3sAXfvw
You now have all the tools you need, I maxed tailoring second because I got lucky and could craft my endgame armor, if you don't want to do it just yet put the points in electronics for better shields. You should be able to figure out what you want to max next, I recommend finishing the crafting skills before the Deep Caverns but Biology could wait until you have supersoldier drug. Now it's also a good time to start putting some more points in stealth.

For the level 30 check the sample build.