Buy Magic Cross from the demon just beside the dark castle. Talk to him and he will say 'I can awaken your demon's hidden powers' then 'Select your guide'. There is a fine but it is worth it. Buy the magic for Gale (who should be in his final form), then go through the castle. When you get to Sythfer, just keep using Magic Cross. It rarely misses and does about 150 damage each time. It attacks eight times; for about 900 total damage each time. He changes form frequently and does not have a health meter. Just keep attacking with Magic Cross and your other best attacks. He should fall in about fifteen turns.