This is only if you don't want to cheese your way to max skill level!
The requirements for this are to have an attribute(Cool,body,Reflexes, etc.) to level 20 and one of the Skills related to said attribute to be level 20 as well. Your skill level will not go beyond your attribute level, example Handguns will max out at 10 if your attribute Reflex is at 10 and will not gain any extra XP until you upgrade your reflex to 11. Perks are not a requirement although you can buy a Tabula from rippersdocs to respect your perks if needed for a low fee of 100K Eddie's.
For my playthrough I maxed out Cool and went with stealth.
The problem for myself was that I played on easy which means I didn't get as much XP after completing every side gig and NCPD scanner hustle in the game and at the end I was only level 17 stealth.
Keep in mind these numbers aren't at all accurate which is why I'm giving ranges.
To reach level 20 on your skill it will take around 500,000 Skill XP not including leveling up attributes. You get a set amount per kill/action on what difficulty level you play on. Since I was on easy I got around 80-90 per stealth kill and double if I dumped the body in a container/fridge/dumpster. Playing on very hard will grant you at least double that if not a little bit more.
With that said I had to grind from level 17 stealth to 20 and it took some time but would've been way longer doing this on easy so I recommend on very hard. Also keep in mind I was heavily specced into stealth so from stealth with an upgraded revolver I one shotted every enemy headshot or not. This route took me 10-15 minutes eventually when I got it all on routine and netted me 4-5K skill XP per run.
method for attribute levels and skill levels
This grind spot I used is really only for people who are in my position and who are trying to max out these skills:
If you've maxed reflexes attribute:
If you've maxed out body attribute:
If you've maxed out cool attribute:
Intelligence attribute may work as well for quick hacking and breach protocol but may take longer
Now onto the grinding spot: Pacifica dataterm by the Grand Imperial Mall contains a clockwise loop I took with about 30-35 enemies with plenty of places to dump bodies if you're doing stealth.
1.When you spawn at the dataterm to your left will be three enemies. These respawn on a short timer.
2. To the right of them is market area a few metres away. This area may or may not respawn the enemies when you leave the Pacifica but won't hurt to check.
3. Turn around and head inside the Grand Imperial Mall. There are 8-10 enemies here, they usually respawn after leaving Pacifica via dataterm and coming back. Also, lots of places to dump bodies.
4. Head out the mall to the left you will see three enemies firing at a turret. These respawn on a timer like the other 3 near the dataterm.
5. Across from the highway jump up to a small market area with 6-8 enemies.
6. Now head back to the dataterm but on the way the you will see a construction site to your left with another 6-8 enemies and places to dump bodies.
Every stealth kill granted me 188XP and double if I dumped the body in a container. One other note on dumpsters/containers, once you fill them up you can't again for some reason they stay full so after awhile you will fill all of them within this area and can't use them again for some reason.
Below is a list of skills and how to get XP for each
Athletics: Dash, Dodge, Climbing, Jumping
Annihilation: Using Shotguns & LMGs
Street Brawler: Using melee blunt weapons
Assault: Using SMGs, Assault Rifles, & Sniper Rifles
Handgun: Using Handguns & Revolvers
Blade: Using Blades
Crafting: Crafting and dissasembling items
Engineer: Using Tech Weapons or smart guns
Breach Protocol: Doing breach protocols
Quickhack: Using quickhacks in battles
Stealth: Stealth Kills, Successful Takedowns, Surprise Attacks, dumping bodies in containers/freezers/dumpsters
Cold Blood: Using bladed weapons