The last mimic chest is in the Volcano Fortress. At the start of the dungeon is a door that you need to beat two bosses to open. Go through that door, then one room further. On the right side of this room will be a broken wall, which leads to a room with the last mimic chest. And just FYI, you don't have to worry about any points of no return. There's only one of those at the very end of the game and there's a text warning before it happens.