This tutorial explains how you can find Shiva in Final Fantasy V Pixel Remaster. You can only get this summon in Bartz’s World. Before you leave the first world, you need to go to Castle Walse.
On the second floor, take the steps down on the right side. You then need to take the southeast exit on the first floor.
When you are back outside, look for a secret entrance in the waterfall on the north side.
In the Water Tower, make your way to the fourth floor. You should find Shiva in the middle.
During the fight, use any fire spells and attacks you have. Once you defeat Shiva, you will be able to summon her in battle.
Afterwards, you can take the south exit to quickly leave the Water Tower.