How to quickly gain experience?
Slow experience gain is a quite serious problem in the beginning of Witcher 3, especially on the highest difficulty level, where Geralt has not many free skill slots. Luckily, there is a way to quickly gain experience. It is based on killing drowners under water. To do it, you will need a crossbow (the one you obtain from Vesemir in White Orchard will be enough, but the stronger, the better) and some patience. The method described below works well in version 1.02 of the game. It is possible, that one of future patches will remove this exploit.
Crossbow is not a weapon that makes you much stronger when fighting on the land, but under water situation is quite different. One or two hits are enough to kill almost every swimming opponent. It makes this method rather safe - before enemy gets close enough to deal a hit, Geralt will be able to eliminate him with no trouble. The key for the operation to success is finding a good place to act. It is best to shoot at enemies near the shore. Thanks to that, if something goes wrong, you will be able to quickly run to the land. Drowners usually don't walk out of water in order to chase the witcher so you shouldn't worry that the battle would continue on the land.
The game spawns creatures rather quickly. Thanks to that, you can attack enemies endlessly, only stopping to get some fresh air. For the drowners to respawn you must perform a full rotation of the cameras. Territories directly to the west from Crow's Perch are perfect for the hunt (more precise location can be found on the map above). It's a place near shipwrecks. Exploit can be used in other parts of the game world as well - you must only find an water area in which Witcher 3 generates large amounts of drowners or sirens. Interestingly, while eliminating enemies you don't need to worry about where your bullets are going. Geralt automatically hits each enemy that is in the range of camera. You don't need to set enemy aiming or perform manual aiming.
Numbers are the best witness of the efficiency of this method. For example, in Velen Geralt is able to gain level in less than ten minutes. In this location drowners can be found on tenth, much higher level. The amount of experience gained differs depending on difficulty level, precise values for example mentioned above are following:
Geralt on 7 experience level versus drowners on 10 experience level:
Difficulty level I
Difficulty level II
Difficulty level III
Difficulty level IV
9 XP for a kill
7 XP for a kill
6 XP for a kill
6 XP for a kill