Here's what I did to get extra XP and resources; I went out in the forest and killed nekkers and endregas without doing the monster hunting quests themselves. This way there would be a huge amount of them and they would keep respawning at sure locations. This way you could get a ton of crafting equipment and like 2-3 levels worth of XP before the monsters themselves wouldn't give more XP. This trick can make the first act significantly easier (especially the second boss fight).
I have another one, but it's more like exploiting; During the second act, if you go into the mist you'll get one-hitted by either an arrow or a sword. Thing is, if you have improved dodge, you can quickly roll to the small valley (under the stone bridge) without getting hit. At this place, the only way archers can hit you is if they start spawning (this happens after around 5 minutes). For this trick to work, however, you need to avoid all their attacks as even one hit will likely kill you. After you're satisfied with the XP you got (you can get 300-400 with one run) or the archers start spawning around you, roll to the fog at the Vergen side and it will teleport you to the other side, where you have to do 1-2 rolls to get out of the fog. Doing this can get you to maximum level by the third act, though it's not easy at the beginning, so don't wonder if you won't even get to the small valley and end up being a hedgehog in the middle of the battlefield.

I didn't, but when going the Roche's path, there's the hut with the old man making candles within and when you're outside the circle of candles, harpies keep attacking you - they're spawning infinitely and it's very easy to just stand where they spawn and dispatch them using heavy attack repeatedly.