Golden Sun: The Lost Age/Kandorean Temple
You'll notice that the monks at the front gate won't let you past, so to get around this run around the left-hand side of the wall. At the end, Kraden will remark on a suspicious piece of ivy. If you've played Golden Sun you'll know exactly what to do here - get Sheba to use Whirlwind on it to scatter the ivy and reveal a cave entrance. Follow the watery path down here, and climb up the rope at the end. No-one seems to mind you're there though, so go ahead into the temple.
Master Poi will be up on a high ledge, with three trainee monks practicing their levitation skills. All three of them seem to be pretty bad at it, but the one in the middle loses his patience and wants to enter the cave. Master Poi lets him, which turns out to be a mistake. Anyways, there's three of you, so you'll be fine. Enter the cave.
 Inside the Cave
This is your first proper dungeon. The enemies down here aren't particularly tough, but the layout may make it somewhat hard to navigate. Go straight on and open the chest. It's a Mimic! Mimics will appear as treasure chests and you can be certain that there is one in every major dungeon. They're considerably tougher than standard enemies, but you get EXP as well as a relatively decent item off them when they die. If you haven't summoned Venus already, do so with Felix, or unleash Echo, or attack. Jenna should either attack or cast Fume, and Sheba should cast Ray. Repeat until the Mimic is dead. For the mean time it is of critical importance that you don't let any of your characters die, because for quite a while resurrecting characters is a costly and arduous process.
Once it dies, you get a Game Ticket (which is useless for the mean time). Go back and head left. Follow the long path around to the stairs down. You'll emerge in a very large room filled with rock pillars. From here, none of them go anywhere, so go down the ladder. Go to the left and weave in-between all the pillars. If you can't find a way through, there's a worn-away path leading through to help you. Climb the ladder and follow the ledge around. Jump on to the first pillar you can, and then jump across until you reach a door to the north.
In the next large cave there's only one way you can go, so head to the left and down the stairs. In this long room, head down the ladder and down the left-hand side of the flowing river. When you can actually get into the river, run all the way back up it, around all the rocks. Get off at the right-hand side near the top, and weave back down this side and around to another doorway.
It's a good idea now to assign Move to either of the shoulder buttons. Go into your Psynergy menu and press either L or R, then selected Move from Felix's Psynergy. Try to push as much for the mean time though, because when you'll need Felix to use Psynergy in battle, he won't be able to if you've used Move too much. This just makes other parts of the game (when you might be pressed for time) a lot easier. Anyway, shove the moveable pillar on the left hand jet, then get on the other one. When they'd ordinarily go up, the pressure makes the right-hand platform go extra high (up to the next floor). Up here, grab the Mysterious Card from the chest (equipping this will make your party members a unique class, but it's not that good) then hop back on the platform.
Back down in the jet room, move the rock pillar on to the jet you just used, then get on the left-hand one. When it goes up, jump off at the top, and head left to the ladder. Climb up, and walk across the tightrope to the next platform. Don't cross the next one just yet - the jet of gas will push you off every time. Instead, go up to the moveable pillar. Use Move to push the pillar away from you, then jump across and shove it along until it blocks the gas jet. Now you can use the next tightrope to get across. The ladder here is deceptive - you can't get past by going down it. Instead, go around the ledge to the right and then you can jump between the wall-ledges further down to get past the rock. Go down the ladder and exit the room to the south.
You'll recognise this room. You can't do anything with the rope here yet, so go downwards. Exit through the door, and you'll appear in the fenced-off section of the dungeon you came past at the very start. Head up the other set of stairs. Go up to the ladder in the next room, and you'll see a weird cauldron of sorts blocking yout way. You can't push this (I assume the monks would have to concentrate) but you have your good friend Move, who can touch it with no harm to you whatsover. Move out of your path and jump across. Before you go any further, make sure your characters are healed and Echo is Set. You'll want to unleash it in the battle up ahead. When you're ready, keep walking up to the three monkeys blocking your way.
 Boss Battle: Chestbeaters
Your first and easiest boss battle. To begin with, have Felix unleash Echo on the middle Chestbeater, Jenna cast Flare or Flare Wall if you have it (with the big arrow on the middle Chestbeater) and Sheba cast Ray (also on the middle one - this is focussing your attacks to hit every foe, and something you'll have to consider when using Summons and Psynergy). Your foes are weak to Mars (Fire), so Jenna should be dealing a fair amount of damage. When Felix can summon Venus, focus it on the middle Chestbeater. Repeat this (Felix can attack) until they're all dead.
When you beat them, climb up the ladder that appears and follow the path around until you reach Master Poi's room. He'll think it was the monk, but he's surprised when he sees you are there! After a discussion, you can climb up the rope to reach the funky rock thing on the plinth at the top of the room. You've now acquired Lash! If you speak to Master Poi, he'll tell you there's something you might find interesting in the cave. Remember that rope you passed before? You can now go and use it. Equip the Lash Pebble on Sheba, then backtrack your way through the cave until you get there. Use Lash and climb up to reach a Mercury Djinni. Why this is here when you don't have any Mercury Adept to give it to, I don't know, but you'll have to fight it if you want it. A combination of Felix's Spire, Jenna's Fume and Sheba's Ray followed by normal attacks should finish it off. You can now use Fog the Mercury Djinni to summon Mercury, but it's probably best if you leave it set to Jenna for the mean time.
Now leave the cave and Kandorean Temple. On the world map, go back to near Daila (you can pop in and use the Inn or buy equipment if you need to). When you're ready, head east from Daila and keep going until you reach a cave in front of a red tower.