Right before the mission of Timber. Speak again with Cid and he will give you a magical lamp.
In this Lamb, there's Diablos, Diablos could be a bit difficult if you fight with him with low HP and with low weapons. But there's a easy way to defeat him, put draw in all the 3 characters and try to draw the ???? magic. That's gravity, and if you are lucky with 2 nice extrations you could do a lot of damage. Once you realized the extration is taking low HP to Diablos, then switch to some GF like Shiva or Ifrit, or just physical attacks. I recommend using GF cause he has low HP alredy and he start attacking a lot.
Hope this helps. Good luck.