honestly, your lvl is too low, it's not like it's impossible, but with lvl 12 it's not worth it.
instead of gris I recommend to use Hirge with intervention, also lot of mana potions. also a good advice is to use ice elemental scroll as meatshield, makes it easier to fight that dragon.
but to be honest, it's not worth for you to kill that enemy now, you have no gold to buy new skills and also your lvl is just too low to use new build effective