It has been quite a while since I have played OOS, but I think that I do recall that you have to go up to where Onox's Tower is. Jump over the set of 3 holes near the guy spinning the mace. You'll see a warp right above you. I can not initially remember how to get to it, but once you do, go through it and into Subrosia. Inside the cave, throw a bomb into the lava pool? The volcano will then erupt, spreading lava all over the ruins.
When you get back to Hollodrum, change the season to Summer and go right on screen. There's 2 lynels and a vine, climb it. Jump over the lava onto the patches of Earth. After that, jump over the lava and into the cave. I do not remember a thing about the cave, so once you get through it, you will be on top of a tower with two lynels close by. Go right and into another cave, you will come out on the other side. Go into the warp and back into Subrosia. Go north, i think, two squares and into the glaring volcano. Welcome to the Sword and Shield dungeon.