Upon entering the Temple of Fiends you can see Garland's room right above you. You prolly should raid the treasure chests first.
You find a cap. Strap that sucker on to somebody! The cap is one of only 2 helmets that every class can equip.
A heal potion! That'll be useful, especially if you didn't buy any.
Ooh, a cabin. These are fine to get from a chest but don't waste your money on them. They do twice what a Tent does, but cost more than twice as much.
Blast! The Mystic Key strikes again.
When you enter Garland's room you see his 5 pet bats, Princess Sara, the Black Orb, and Garland himself. Use your CURE spells and heal potions to get up to full HP. Some people may say to save your spells for battle but think about it: A heal potion restores more HP than CURE and anyone can use it. So use your CURE spells first! As a general tip, if you are not in a dangerous situation, use magic first because it is cheap to renew but potions are expensive (and take too damn long to buy). If you're not confident you could use a tent outside to save and recover some HP.
You impertinent fools, he will knock you all down!! The battle is really easy, depending on level. If you are in level 1 the battle may actually be difficult, 2 will be easy, and 3 & up will be very easy.
Beating the G-man is easy. Fighters, Thieves, Black Belts, and Red Mages should fight. The White Mage should either heal party members or fight, as the situation demands. The Black Mage should use FIRE or LIT.
Hmm, something about these bats seems suspicious...
Once you've saved the princess, she takes you back to Coneria Castle...