Golden Sun: The Lost Age/Indra Cavern
This is where the first Combo Tablet is. Combo Tablets will allow you to gain powerful Summons that weren't in the original game. This one is Zagan. To get it, Move the pillar surrounded by rocks across one and use Lash on the rope. Climb up, hop across, and use the Tablet. A load of runes will appear, before entering Felix's head. You can now summon Zagan! It requires one Venus and one Mars Djinni on standby, and is a little more powerful than any of the Summons you can use so far. It may also lower Defence slightly. Now, leave the cavern and head west again. Head straight across the forest and follow the path south, until you see Madra.
Iron, the Venus Djinni
There's a large forest with three small circular clearings here, just to the left-hand side of Madra. Wander around in here. Eventually, Iron will show up in a random battle. Defeat it like you would any other foe, but just remember that Sheba may take a bit of a pounding in this battle since it is a Venus Djinni and is very fond of casting things like Growth and Spire. If it runs, just keep wandering around until it reappears. Once you've defeated it, it'll join your party (make sure to Set it and put it on Felix). Once you've done that, enter Madra.