Pathfinder Kingmaker build: Eldritch Archer merc

stupid damage in action: https://www.youtube.com/watch?v=L-6-S9k_94I

Elf, Lawful Good for weapon enhancements
This can also be done with a Tiefling, but what's more elven than longbows and spellcasting?
Str: 14
Dex: 18 -> 22
Con: 12
Int: 16
Wis: 9 -> 10 at 20
Cha: 7

EA - Eldritch Archer (yep, no multiclassing in this one)
While I'm listing which spells to pick, they can also copy spells from scrolls. But since their list of useful spells is so limited to begin with, there is barely any point in copying anything.
1  - EA - Point-Blank Shot // Burning Hands, Color Spray, Grease, Magic Missile, Shield, True Strike
2  - EA - Snowball, Enlarge Person
3  - EA - Precise Shot, Arcane Accuracy // Expeditious Retreat, Vanish
4  - EA - Scorching Ray, Web
5  - EA - Weapon Focus > Longbow, Rapid Shot // Acid Arrow, Glitterdust
6  - EA - Enduring Blade // Blur, Mirror Image
7  - EA - Dazzling Display // Fireball, Stinking Cloud (swap Fireball with Haste or Displacement if one is lacking in your party)
8  - EA - Haste, Displacement
9  - EA - Manyshot, Ghost Blade for main campaign/Empowered Magic for DLC dungeon // Lightning Bolt, Slow
10 - EA - Controlled Fireball, Greater Invisibility
11 - EA - Shatter Defenses, Metamagic Heighten for main campaign/Maximize for DLC // Dimension Door, Stoneskin
12 - EA - Devoted Blade // Dragon's Breath, Ice Storm
13 - EA - Clustered Shots // Acidic Spray, Cloudkill
14 - EA - Cone of Cold, Fire Snake
15 - EA - Improved Critical > Longbow, Bane Blade // Baleful Polymorph, Invisbility (I guess?)
16 - EA - Hellfire Ray, True Seeing
17 - EA - Arcane Strike, Greater Weapon Focus > Longbow // Disintegrate, Umbral Strike
18 - EA - Maximized Magic // whatever spells
19 - EA - Toughness // pick 2 whatever Magus spells and then you get to pick Wizard spells - Hurricane Bow and Sense Vitals are useful for sure, rest is up to you (maybe Echolocation, Communial Stoneskin, Animal Growth can still be useful)
20 - imagine if EA had a good level 20 talent. So just pick whatever you like (draconic Sorcerer for bonus fire damage? Vivi is probably solid as usual for 1d6 Sneak Attack and 10 min mutagen)

Eldritch Archer is a class I wanna try for a merc, so I wrote down this build to check out what they can (and can't) pick up.
Update: I have since playtested this build on a merc in the DLC dungeon, I'll write down my thoughts at the end of these notes.

Eldritch Archer is a bit of a weird class. It undoubtedly has an early power spike with its weapon enhancements in the early levels, and it can keep shooting those ranged touch spells for bonus damage on top of the massive damage that longbow users do by default. There is just one big problem with it: The class barely has any unique features tailored towards ranged combat (medium and heavy armor proficiency anyone?), and the Magus spell list... flat out sucks for what this class wants. Sure, it can pick up spells that are really, really good to have early on - Grease and Web are great disables, Stinking Cloud is amazing and you can't ever go wrong with Fireball and Controlled Fireball. But a couple of levels after getting those, the spells quickly lose their value, and you simply don't have any ranged touch spells to use instead. Level 5 spells are especially garbage.

Metamagic is decent to work around this, but there also isn't a single one Metamagic feat that catches it all. With Heighten, you can simply throw Scorching Ray into your level 3-5 slots and call it a day, but god forbid that you'll ever meet enemies that have energy resistance or are completely immune against fire. Acid Arrow is a 'meh' replacement at best.
It might be prudent to use Maximize instead. You could maximize level 1 spells to 4 (maximized 5d6 Snowball is still 30 damage and suffers less from damage reduction than Scorching Ray), and Scorching Ray could be maximized into the level 5 slots (24*3 fire damage damage). This would still leave you with no ranged touch spells for level 3, but these slots could be filled up with Haste and Displacement. These two are always good.
Empower would help you fill levels 3 and 4, but leave you with the gaping void that are level 5 spells. I feel like this would be the worst option of the three.
Maximize seems like the best option to me, but this would have be tested in a proper playthrough to really experience.

On the topic of multiclassing, I don't really see how this class would benefit from it. You certainly want those 13 points for weapon enhancement +4 as the class has access to tons of good enhancements. Devoted Blade at 12 and even Bane at level 15 are both wonderful to have as they work with your spells. But since you're already so deep into the game at this point, what could you possible multiclass into? Dragon Disciple with its strength bonus would merely be a small damage bonus since we're ranged, Arcane Trickster kinda sucks with its minimal base attack bonus, and Eldritch Knight offers nothing special since we'll never get those 10 levels into it. So you might as well stick to Eldritch Archer, even with its reduced BAB progression.

Skills points are another awkward aspect. Using a Magus rather than a Ranger or an Inquisitor for your ranged DPS means that you'll have fewer skill points available, even with their higher int modifier. You'll only have skill points for 3 skills + a half leveled one. But what's worse is that the dex skills (Mobility, Trickery, Stealth) aren't class skills for Eldritch Archer, while the Knowledge skills are. This is just awkward because obviously you're gonna put your attribute points at 4/8/12/16 into dex and not int, so you get to choose if you want to be mediocre at dex skills... or be mediocre at int skills. You'll definitely want to bring an int character with a Skill Focus on Trickery to make up for the lack of competence on this one. You can also skip a combat feat after Shatter at 11 to grab a Skill Focus as well if you feel like you need it.

Update:
Yeah, Eldritch Archer is an excellent glass cannon ranged DPS. It adds little in terms of utility, but its damage potential is enormous. Once you get Devoted Blade at level 12, Chaotic Evil enemies aren't even allowed to exist around you anymore. It's basically a ranged Paladin on crack with more versatile weapon enhancement options.
On that topic, Ghost Blade really isn't all that great. Later enemies don't ever wear armor, and hitting those spectres is a bit of a specific use case. It's still worth taking for the main campaign, though, since the only alternative to that is Empowered Magic. That one is better for the DLC dungeon for sure.
Maximize Metamagic turned out well. Heighten for more Scorching Rays would certainly be stronger in an ideal setting, but getting to choose between maximized fire or cold damage is very convenient since you can simply choose which one of those you want to use in combat... without ever adjusting your spellbook. For the main campaign, though, you'll be facing a lot of enemies in the late game that are simply cold immune, so maximized Snowballs won't get you anymore. I think I'd prefer Heighten there.
The Wizard spells at level 19 are another serious power spike, too bad that's so late into the game. Once you have those, this class suddenly gets super stacked.

Long story short, Eldritch Archer is a great choice for ranged DPS if you're not looking for additional utility.