For this achievement you're only required to level a single skill to 100. Which sounds more time-consuming than it actually is, considering how quickly most skills level and all the glitches, exploits, and boosting and grinding techniques there are in the game, some of which I've included in this guide.
In Skyrim, the player has a level for each individual skill along with a character level that increases after a set number of skills increase. You can level a skill by preforming an action related to that skill (e.g. killing enemies with a bow to increase the Archery skill), spending gold to train with an NPC, reading skill books, or gaining skill increases as a quest reward.
Attached to this guide, are various videos showing the techniques described. If you decide to skip to the videos, I'd recommend you at least read General Leveling Tips.
If any of the below methods stop working due to a patch or something, leave a comment. I'll alter the solution.
Complete list of Skills:
There are 18 skills in all. They are split up into there categories: magic, combat, and stealth.
This is the complete list of skills:
General Leveling Tips:
A good way to make leveling go by faster, is to activate one of the three standing stones located at The Guardian Stones, which is Southwest of Riverwood and Northwest of Helgen. There are three standing stones here, the Mage, Warrior, and Thief stones. Each stone multiplies your skill gain from one of the three different skill categories by 20%. You can only have one stone active at a time, but can travel here to change to a different stone any time you want.
Sleeping provides resting bonuses that improve skill gain for 8 in-game hours. The sleeping bonuses you receive depend on where you sleep, such as:
• Sleeping in a bed you don't own grants you the Rested bonus, which multiplies all skill increases by 5%.
• Sleeping in a bed that you do own grants you the Well Rested bonus, which multiplies all skill increases by 10%. This applies to beds in player-owned houses and to beds that you have rented in an inn.
• Sleeping in a bed in the same building as your spouse grants you the Lover's Comfort bonus, which multiplies all skill increases by 15%.
Enemies level as the player levels, so leveling too many non-combative skills too quickly can lead to a character weaker than his character level.
The Legit Way:
Skyrim is a good game. If you just play it, sooner or later, this'll pop. You're going to have to play it a lot anyways, if you plan on completing it, so it's not really necessary to grind.
I'm not gonna get all preachy and tell you that boosting is wrong and you shouldn't do it, though. There's nothing wrong with boosting and this is a boosting site. Besides, I've boosted lots of games myself. It's your choice how you choose to game.
Oghma Infinium Book Exploit:
With this you can level any or all skills to 100 with minimal effort.
For this, you need to be a level 15 or higher and own the house in Whiterun with the living room decorations purchased.
Travel to the secluded cave Septimus Signus's Outpost, north of the College of Winterhold, to meet a recluse named Septimus Signus. He'll give the Daedric quest "Discerning the Transmundane."
Note: This quest is concurrent with the main quest "Elder Knowledge." Completing it before "Elder Knowledge" will glitch this main quest mission. The main quest can still be completed, though, but you'll be forced to skip some minor dialogue.
Completing this mission will give you the Daedric artifact, Oghma Infinium.
This is a book that levels all the skills in one skill category of your choosing. If you use it, though, it will vanish. This exploit will let you use it infinitely without it vanishing.
If you don't own the house in Whiterun, play the main quest until you complete at least "Dragon Rising", then talk to the court mage, Farengar Secret-Fire to recieve the quest "Bleak Falls Barrow", which after completion will allow you the purchase of the house, Breezehome, located next to the blacksmith.
Purchase the house, buy the living room upgrade, and enter Breezehome. Now, complete these steps:
1. Activate one of the bookcase shelves.
2. Select the Oghma Infinium in your inventory and press either RT or LT to read it.
3. Select the skill category you will to level and press back once.
4. Place the book on the shelf and exit.
5. Activate the book, select "(Do not read)" and take the book.
6. Repeat all steps until skill(s) is/are maxed.
Black-Briar Meadery Speech Exploit:
While talking to a specific vendor in Riften, you'll have the option of persuading him into giving you a little insight. Do to a little error in programming, you can persuade this character multiple times, with each persuasion granting Speech experience.
Travel to Riften and enter Black-Briar Meadery, then follow these steps:
1. Speak with Ungrien, the man standing behind the counter. If he's not there, wait an hour or two.
2. Select "Tell me about Maven Black-Briar."
3. Select "Come on, get the truth off your chest. (Persuade)" or if your Speech is lower than 25, select "Here, now tell me the truth. ([small amount of] gold)".
3. Repeat all steps until the Speech skill is maxed.
Note: After completing the Thieves Guild quest line, Ungrien may disappear, and be replaced by Romlyn Dreth.
By accumulating leather strips and iron ingots, you can smith lots of daggers, quickly increasing the Smithing skill.
A good place to do this is Whiterun. Travel there and speak with Eorlund Gray-Mane at the Skyforge outside of Jorrvaskr (the Companions headquarters), and buy all the iron ingot, iron ore, leather, and leather strips that he has.
Then go to Warmaiden's and speak with Adrianne Avenicci. She's the blacksmith in front of the shop between 12pm and 2pm. Make sure to buy from her when she's outside, as her inventory will be different from the shop's. Buy all the same items from her, then, enter the shop and buy the same from her husband, Ulfberth War-Bear.
Now, exit the shop and get started making dagger. You can use Warmaiden's forge. Use the smelter to smelt as many iron ingots as you can, and the tanner to tan any hides you may have, then turn all leather into leather strips.
Activate the forge, select iron, then highlight iron daggers. Rapidly click A to continuously smith until your resources are depleted.
Again, those steps are:
1. Buy all the iron ingot, iron ore, leather, and leather strips that you can.
2. Smelt iron ore into iron ingot, tan any hides you may have into leather, and turn all leather into leather strips.
3. Smith iron dagger repeatedly until resources are depleted.
4. Repeat all steps until the Smithing skill is maxed.
After smithing the daggers, you can sell them if you'd like, or you can enchant them (preferably with the Banish enchantment), then sell them.
Some spells can be cast on yourself at anytime. Muffle is one of these. It gives large quantities of experience for Illusion.
You'll need to be at least a level 25, and have purchased the Muffle spell. If you can't find someone to sell you the spell, try one of the court wizards, such as Farengar Secret-Fire in Whiterun's Dragonsreach.
You can improve your Magicka regeneration, increase your Magicka, and decrease Magicka usage by equipping enchanted items and using spells.
The more expensive an item you pickpocket is, the more experience you'll gain. To make pickpocketing easier, fish for items that weigh less. Jewels and Jewelry are easy and net lots of experience.
This exploit will let you pickpocket the items off of a specific NPC, then reverse-pickpocket the items back onto him, and if he catches you it won't summon the guards.
Travel to Riften, then commit a small crime. Pickpocketing a guard is a good option. Choose to do jail-time, then while in jail, lockpick the door. Now, sneak to the cell across from yours. Inside it is Sibbi Black-Briar. Complete these steps:
1a. Pickpock Sibbi. Take the most expensive items he has.
1b. If you get caught repeat step 1a.
2a. Reverse pickpocket Sibbi. Put the most expensive items you have on him.
2b. If you get caught repeat step 2a.
3. Repeat all steps until the Pickpocketing skill is maxed.
Keep a lookout for guards. They may not be alerted to you when you get caught, but they still wander around. And, if they catch you, they'll kill you. Be sure to save every now and again to be safe.
The below Youtube videos are shown in an order compared to the order of the above sections respectively. There not my videos and I don't take credit for them.
The "Oghma Infinium Book Glitch/Exploit" and "Max Speech Glitch" videos were posted by Gametastik.
"Tutorial to Easy 100 Smithing" was posted by stevenwg19.
"Level Up Illusion to 100 fast" was posted by VesperAzure.
"How to Level Pickpocketing to 100 Fast! " was posted by Wiggmuzzle.