Speaking of the promotion items, where do you obtain the Earth Seals? I need to promote my Myrmidon, but I don't know where to get the Earth seal for them.
I never understood what a high Constitution means, could you explain it?
See that Con stat? If it's high, that's a good thing. In the GBA Fire Emblem games, your unit's Constitution governs what weapons they can safely use. Every weapon has a weight, and if that weight is greater than your Constitution, the difference is subtracted from your speed. Mounted units can get around this somewhat, but they still need to watch out.
A general rule of thumb: Slim weapons are worthless, Iron weapons are awesome, Steel weapons are decent, Killer weapons are amazing if you're willing to ration its uses, Silver weapons are completely pointless (too strong for normal enemies, usually a liability against bosses), and the special weapons are great but usually too expensive.
Also, if a unit has higher Consitution than another, the latter can be rescued by the former. The "savior" unit gets his speed halved while he's carrying someone though.