Get your intelligence levels as high as you can, INT is tied to X, on double XP events I get my INT special to well above 40. Get 4x Lunchboxes for 100% XP. Egghead serum for +6 INT, Max INT legendary perk card, Max chem fiend card with Berry Mentats (+5 INT for 5 mins), cranberry relish for +10% XP, Brain Fungus soup for +2 INT, join a public INT team (casual) and get the Inspirational card and Strange in Numbers card for more XP/INT. Get well rested for +5% XP. Run West-Tek 100's upon 100's of times. During double XP events I can get 2500+ XP for every Level 100 Super mutant kill, the next double XP weekend is on 10th December. Good luck!