The Double XP codes, I'm pretty sure, are for Multi only. Different "training" spots would be the best way to level up without glitching. Run around the map in a circle to kite and bunch up the zombies before you unload on them.
If you go up the stairs on BoTD in the Wardens house with 3 bots on solo and just watch the window, its a AFK zombie farm on Hardcore. The Bots take very little damage and they 1 shot most zombies at round 40+. Its not the worlds highest XP/hr but its a nice little farm. Just watch out for dogs, they will demolish those zombies.