This isn’t easy, but there are a few ways to make the journey a little quicker.
One of the most important methods is to gain the Experienced perk which offers a +10% XP boost. This is achieved by destroying 10 different training dummies. You can do this across a number of locations in the game, but it is best to do it as soon as possible.
The earliest place to get this is in Foltest’s Camp. There are 5 dummies in the area that respawn. Destroy 10 and you’ll earn the perk.
I strongly advise doing this on your easy playthrough. This achievement requires you to complete pretty much every quest as well as kill just about every monster you encounter. If, like me, you missed or failed some quests here and there, or you think you aren’t going to hit level 35 by the end of the game, fear not. First off, the last quest, A Summit of Mages, will net you 1000XP for completing it, so you have to be level 34 before you go through with it (make a save before you do it just to be safe) if you want to get this achievement. Most importantly, there is actually a spot in Chapter III that will allow you to easily grind for XP.
The spot is located just outside the Loc Muinne city, in the camp of the Knights of the Order. These knights wear red armor and you will be able to kill them indefinitely for about 5XP every kill. They respawn almost instantly and most of the time they don’t even fight back, making it ridiculously easy to farm here. Grind here until you reach level 34, and then proceed to complete the last main quest.
IMPORTANT NOTE: Many people have reported not being able to attack the Knights at all, hence not being able to kill them. If you followed Iorveth’s path, you should be able to do this automatically, and they will actually attack you on sight. On Roche’s path, you must help Roche rescue Anais La Valette from Dethmold, and tell him to reconsider giving her to the Order, and to consider his other options. Make a save before you talk with him here, just to be safe. You’ll know you did it right if Roche says he has other plans when the Order confronts him outside of Dethmold’s hideout, because after the dialogue, a combat encounter will occur, and you’ll fight the order. After fighting them here, return to the camp outside Loc Muinne and start killing them.
To minimize the grinding, I would recommend taking on as many side quests as possible and fighting every enemy you encounter without running past it. However you decide to go about doing this, the achievement will unlock once you hit level 35.