The answer is really quite simple: you can’t get it in the USA or European versions of the game. In the original Shining Force game, characters could search during battles, but in the USA and European versions of Shining Force 2, they can’t. The option is available in the Japanese version of the game (when the game was translated, a few features had to be removed so that it could fit on the cartridge), and this chest contains a Shining Ball.
How do I get the chest outside Mitula's Sanctuary - the one on the island in the lake around the temple?
Whenever I go to fight the big guy in armour (The Taros), he just tells me to go away. How do I fight him?
Rohde always tells me that he's busy and to come back later. When I do, he says the same. How can I make him join me?
When I go back to Yeel, after I've been to Parmecia, the town is deserted. What am I supposed to do here?
On the west side of Pacalon Castle, I can see a stairway, but no way to reach it. How can I reach it?
I follow an old man to a room where he says there is a tunnel to Grans Island. When we get there and I kill the monsters, he tells me it's locked. Where do I go from here?
How do I use the promotion items (and can I promote a Paladin to Pegasus Knight / Gladiator to Baron / Vicar to Mastermonk / Sniper to Brass Gunner / Wizard to Sorcerer)?
Whenever I save a game and power-down, each time I go back to play it the save game is gone! Why is this? (non-emulator)