Shiva Summon Location and Guide
In the eastern portion of Waltz Castle you'll find a room with multiple exits as well as a switch on the ground that when pushed will open a hidden door to the north which leads to the water way. This switch actually has nothing to do with the Sidequest for Shiva but it's a really good landmark for those looking for this area!
Once you find this room you'll want to take the southeastern exit, where you see me at in the screen shot above. You'll find yourself in the water way outside of Waltz Castle when you take this exit. Follow the water way around and to the north where you'll find a waterfall, the same one shown in my screenshot below. Enter this waterfall and you'll be in Shiva's dungeon.
The dungeon itself is extraordinarily straight forward. It'll likely be too hard for you though if this is your very first time in Waltz Castle. If the enemies in this dungeon aren't too hard - Shiva most likely will be. Either way, follow the dungeon to its end and inspect the fire that's trapped inside a green bubble to summon Shiva.
Before you start the fight with Shiva, put half your group in the front row and half of them in the back this way Shiva can't hit all four of your party members at once. Her main attack will only hit one row at a time, which makes this a great way to halve the damage Shiva will do to you.
If you're having trouble with this fight, I recommend switching up your group composition so that you have two or three Monks and one White Mage. It helps a lot too if you learn !White on each of the Monks so that anyone can heal if they need to. Start by spamming Kick on each of the Monks until the IceCommandrs are defeated. Once they are, single target Shiva.
With this strategy, Shiva should be a push over for you.