Thornskin

You cause a creature you touch to grow hard wooden skin covered in thick thorny bramble growths on their skin and clothes. 

For the duration of the spell, your AC becomes 16+magical bonuses. In addition, whenever you take damage from a melee attack, the foe attacking takes 1d8+spellcasting modifier piercing damage as thorns shoot away from you at the enemy. If the enemy's attack is a critical hit then roll 2d8+spellcasting modifer piercing damage from the explosion of thorns.