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?

You don’t need that tunnel. Take the wooden panel you find in the room to Ribble, and use it on the tree there (note: you must use the A button to select to use the item. Pressing C will only search the tree). Under the tree you’ll find the Achilles Sword, which you can then take to Rohde in Hassan. He’ll then go with you to the Taros Shrine, who you may now fight (note: only the Achilles Sword can harm the Taros). Once the Taros is defeated you can use the Caravan to travel along the dried-up river and get to Devils Tail (west Parmecia) to see Creed.

Use the wooden panel at the tree in Ribble in Shining Force II