Have three players with the Arcane "Dark" magick. Garuda likes Vossler and will attack him first. While he does that, command all the members to cast Dark. When they all cast Dark it will do about 1,890 damage total. Keep repeating this and Garuda will die in about fifty seconds.

To do a lot of damage to Garuda, cast Reflect on all your characters before the battle. Then, cast Dark with all of them to the party itself. By doing this, Dark will bounce off every character and hit Garuda once each, dealing a lot of damage.