Shiva is an optional boss in Final Fantasy V. The party can find her at the back of Castle Walse behind a waterfall. Upon her defeat, the party can summon her into battle. Shiva is also one of the forms Metamorph can morph into.
Challenging Shiva as soon as the battle becomes available is not advisable as the party is yet to acquire the Summoner job, so even if the player wins, they will not be able to summon her until later.
Having half the party in the front row and half in the back helps. The best time to battle Shiva is after coming back after visiting Karnak as the party will be able to use Fira and summon Ifrit. The player can also break a Flame Rod to deal around 1200 damage to the entire enemy group, killing the Ice Commanders instantly and bringing Shiva down to critical health.
If the player does challenge her as soon as the battle becomes available, they should have two Black Mages cast Fire while the Knights and Monks keep on attacking. The Ice Commanders should be defeated first and once they are gone, the party can fully concentrate on Shiva.
"Paladins" can also make the fight possible early on. If the player has all four characters learn !White (level 1 works fine), use them as Knights equipped with Mythril Swords (stealable from Ice Soldiers), and wear the best buyable armor, they will be able to alternate between Cure and attack to win.
It is also possible to acquire Flame Rods as soon as the Shiva battle is available, rather than wait until Karnak to buy them. The Ricard Mage in Walse Tower occasionally drops Flame Rods, which can be used as an item in the Shiva battle to win with little effort, even with a low level party.
Another possible strategy when battling Shiva before going to Karnak is using Blue Magic. Since Shiva is a level 11 boss, one can use Goblin Punch with any character at level 11 since Goblin Punch deals eight times the normal damage when used on an enemy with the same level as the caster. It is also possible to frog the Ice Commanders using Pond's Chorus to greatly reduce the damage they can deal.
A good idea is to bring a Thief into the fight, since they can steal Mythril Swords from the Ice Commanders.