Most people will probably get this naturally if they're doing Contracts at an appropriate level. Optimally, I recommend making sure you do your Contracts while they are within 6 levels of you (if you're 6 levels above it, and the quest is gray in your quest log, you'll get no XP from it).
If you save all your contracts and are playing on Death March! (which gives less XP), you might encounter a situation where you haven't attained level 35 after finishing the main quests. In this case, the best thing to do is to farm a specific Cyclops. He is on the far east side of Ard Skellig, next to a place of power and the mastercrafted griffon gear.
He gives 51xp per kill on Death March (some report only 33) and 81xp per kill on Easy (according to comments), and you can make him respawn by meditating for an hour. If you're just short (as I was), this can get you there. It only took me about 20 minutes to finish up. It would have taken far longer to pick around trying to do side quests for minimal or no gain (and I'm not sure it would have been possible with them).
Here's a video. It's not mine, but it's where I learned this.