The Arai Cloak automatically enters Guardian fights if you have the damage to defeat them. It can be obtained directly with Diamonds in the Magic Shop or as part of the Adventurer Pack in the Magic Shop. Once obtained, your Heroine will always have this item.
Tailored from Arai fabrics, it allows you to automatically enter Guardian fights you are strong enough to win.
This is a must get. It's the difference between looking at your watch every few minutes just to enter that Guardian battle and letting the game pass hundreds of levels while you're doing other things in real life.
Unfortunately it doesn't seem to take into account items such as Fury Goader or Death Hex, so even though you may be able to win with those items, it still will not enter once you get high enough that your base damage isn't enough to win.