If you like the idea of building the baddest Witcher in Temeria, you’re going to have to do some grinding. To maximize your gains, you should grind by killing enemies before you bother doing any quests, as the monsters will give you less experience the higher your level, whereas quest experience rewards are static. After you reach a satisfactory level (or the enemies stop giving good experience) doing the quests will further increase your level.
You really dont need to do this at all to beat the game, as quests and incidental monster encounters will do fine getting you the basic experience you need. If you want a powerful Geralt who can handle anything the game can throw at him, you’ll grind at some point. If you want a potent character, but you don’t want to waste a lot of time to get one, skip grinding in Chapter 1 & 2 and just do the Chapter 3 grinding. The early-going will be tougher, but you’ll save a bit of time.
Chapter 1 Grinding
In Chapter 1, grind by killing Barghests and Drowners, both of which appear at night. When Barghests start giving you 50 experience (level 6 or so), it’s time to stop. The only trouble you’ll have in this area is avoiding the quests "Racists" up by the river and "She’s No Early Bird" outside of the gates leading to the Country Inn. These quests trigger with proximity, and avoiding them prevents you from making a circut of the map when grinding. Still, it’s no big deal if you complete those two quests and then grind… there’s much better experience to be hand in Chapters 2 and 3 anyways.
To grind, just run around outside of the Country Inn, you can do a circut up to town either way you go. You’ll mostly be fighting Barghests, but there’s a Ghoul who spawns near the houses between the Country Inn and the Royal Huntsman, and groups of Drowners that spawn near the docks. This all becomes much simpler if you brave the Crypt and kill several Ghouls in order to claim the Igni sign-then you don’t need to rely on Flint to start campfires or pay the Innkeeper five Orens to rest.
Chapter 2 Grinding
In Chapter 2 you’ll find a great place to level in the crypt in the sewers, where you can hunt Graviers. These undead will give you 200 or so experience per kill up until level 15. To get access to the Crypt, talk to the "Con Artist" in the Temple District near the sewer entrance (he’ll be there between 6:30 and 8:00). Agree to do a little grave-robbing and he’ll give you a key to the crypt.
Note that level 15 is the suggested amount of grinding you should do to make life easier, but it’s easy, if time-consuming, to reach higher levels here… There is one thing you might have noticed about this area, however… it’s damned dark. You could waste time and money creating Cat potions so you can see, but do the math, four or five Graviers per run, 200~ experience per Gravier… it would just be a colossal waste of resources. Instead, turn your gamma up to 2.00 (in the Options, Video Options menu) and you’ll be able to see what’s going on… at least, enough to grind. It’s ugly, to be sure, and certainly not the ideal way to play the game, but it works. Kill the several Graviers wandering about, ignore the Ghouls to save time, and rest at the campfire for an hour. Wash, rinse, repeat.
Chapter 3 Grinding
The grind in Chapter 3 is far superior to the Chapter 1 & 2 grinding spots in both experience and convenience. In the Swamp Cave you’ll find a pack of Cockatrices which give almost three times more experience than Graviers-factor in the dip in experience gained from Geralt’s level and this gap will only widen, so that at level 30 Geralt would gain only 150 experience per Gravier, but still 675 from Cockatrices-one Cockatrice kill would give more experience than an entire run through the Crypt in Chapter 2.
All you have to do is return to the Swamps, enter the Swamp Cave, and smite the Cockatrices. Head outside and rest at the Fireplace near the Swamp Cave entrance (this time you’ll need to rest 24 hours, rather than just one to respawn the enemies). Repeat until satisfied. There’s really only one dark tunnel to worry about, so no need for a Cat potion or gamma manipulation. The only annoying issues with this run are the Wolves that spawn in the cave (they’re trash mobs by now) and the Drowners, Drowned Dead, Bloedzuigers, and Kikimores outside the cave. The latter give nice experience, but it’s still meager compared to what a Cockatrice kill gives you. If you set it up so that you wake in the morning, however, you’ll only ever have to deal with a pair of Drowners/
Drowned Dead and a Kikimore Warrior outside, and most of the time they won’t even bother you, which greatly cuts down on the frustration. Also, since the Cockatrices in the Swamp Cave generally number from between six to ten, you can easily obtain 4200-7000 experience per run… much better than the 800-1000 in the Crypt we were averaging in Chapter 2.