Or you go a step further xD
Training Dummies 5 is the best way to go xp wise but it takes ages to get killed ^^. Dmg and XP Gain of the second Map need some tweaking.
Eventually you will get to a point where you can one shot a Walking Inventory and you have millions of HP, so you won't be able to die because you will heal long before your HP hits zero. The only thing that could kill you is you dying of old age, in which case it becomes a race against time, not of HP and fighting the Walking Inventory becomes the fastest way to gain XP.