In addition to item duplication glitch, there is another glitch which can be used to farm unlimited amount of XP in no time.
The best thing about the glitch is that it can be used right at the very start of the game after you decide to join the ‘Brotherhood of Steel’ faction. In order to use it, you’ll need to high damage sniper rifle — at least +50 damage is a must!
After joining the brotherhood, head over to the Airship HQ in the Prydwen and speak with Proctor Quinlan. You need to agree on assisting him on patrol and acquire the ‘Learning Curve’ optional mission.
Once you’ve accepted the quest, you need to head over to the deck of the airship. At this point, you need to ensure that you’re playing the game on Very Easy and eliminate the target you’re supposed to guard. You need to do so while crouching to stay hidden and the glitch will end the quest while rewarding your with XP for completing the quest.
After you’ve done so, you need to head back to the quest giver and accept the quest again. Do this as many times as you want until you reach your desired level.
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