Armor is pointless against the witch, since potions only affect your Health, not Armor points, You should keep on eating enchanted golden apples or anything that regenerates the Player health, during that, Keep hitting the witch until it dies. But since you got nothing and if you want to kill it, simply run towards it, dodge the potions and keep on punching it at a corner, the witch will try to heal,so keep on punching until it dies.
First of all, witches are dangerous. I almost died when she saw me. I had only half a heart left! I was lucky to still be alive, but u can dodge the potions. I am too scared to kill one. They will drop many items if killed. Kill them with a bow because a sword u have to be close to the mob. And u wanna stay away from her. Keep track of ur health and hunger if u get a tiny hungry then eat something, u will heal a bit faster good luck!!!!
Anyways, Armor is useless when fighting a witch, well, you know, because the potions only affect your health. (poison, damage, etc.)
A recommended thing to have when fighting a witch is Golden Apples, or anything that can regenerate your health fast. And bringing a sword that its attack damage is hight (Stone(?) Sword, Iron Sword, Diamond Sword)
If you're out of stuff to regenerate yourself and/or you break your weapon, just...