Callisto

Callisto is located in the wilderness and you will lose items upon death.

Preparation

Callisto is in level 40+ wilderness so having quick escape methods from PKers are a must.
One method is to purchase a Royal Seed Pod from the PVM Store for 650 points. Clicking on it will teleport you out of the wilderness below level 30.
For fastest kills equip your best ranged gear with a charged Craw's Bow. You can use melee with Viggoras Chainmace and magic with
Thammarons Scepter if you wish.
The weapons listed above have a very effective passive effect against monsters in the wilderness, however regular weapons can be used for slightly lower kill times.
In your inventory take about 6 Overloads/ range potions, 12 Super Restores, 9 Saradomin Brews, and a Royal Seed Pod.
Play around with food/ potion quantities and figure what combination works best to extend trip times based on your levels and gear setups.
It might be a good idea to take some tick-eating food such as Angler Fish -> Karambwans incase of Pkers.

The Fight

Pray magic until you're in Callisto's melee range, then swap to protect from melee. You will take zero damage this way.

Rewards

The most notable drop would be the Tyrannical ring to create the Ring of Wytheria (i)