allright, you can take the Titanite Demon easily with a bow... get a longbow and some 100 arrows (just in case) and run past him at the door that leads to Darkroot Garden. Now he will come as far as the stairs and he will stay there shooting lightning at you. Just target him with R3 and move left<->right at the door (from behind) , stay at the left a while for example so he shoots at you , when he does that you move right and fire an arrow... if you hit him for ~30dmg it will take 25-30arrows IIRC and he will be gone forever... but there are others to kill too, they drop precious Demon Titanite upgrade material
I forgot to mention that IIRC you can't summon at that specific post.... the whole blacksmith area is out of limits