Dragoon Class Djinni Combination
The Dragoon Djinni Combination is available to your Venus Adepts (Isaac and Felix) as well as your Mars Adepts (Garet and Jenna). This class has high attack but specializes mostly in defensive capabilities; boasting good defensive stats as well as the ability to heal other people in your party.
During Golden Sun: The Lost Age you'll also be able to become a Dragoon on your Mercury Adepts as well. I'd recommend you avoid becoming a Dragoon on your Venus/Mars adepts unless you're put into a situation that only has you playing a single character. An example is the Colosso event during the first Golden Sun where you play only Isaac.
As with all the other Tri-Elemental classes, the combination of Djinni that you need varies slightly depending on the character that you wish to become a Dragoon on. Let's take a quick look below at the different combinations you can use to become a Dragoon/Templar/Paladin.
Venus Adepts: 3 Mercury; 3 Mars (Dragoon)
Venus Adepts: 4 Mercury; 4 Mars (Templar)
Venus Adepts: 5 Mercury; 4 Mars (Paladin)
Mars Adepts: 3 Mercury; 3 Venus (Dragoon)
Mars Adepts: 4 Mercury; 4 Venus (Templar)
Mars Adepts: 5 Mercury; 4 Venus (Paladin)
Mercury Adepts: 6 Venus; 1 Mars (Dragoon)
Mercury Adepts: 3 Mars; 3 Venus (Dragoon)
The Templar class has higher stat bonuses than the Dragoon class and following this same logic; the Paladin has the highest stat bonuses of all three. If you decide to push yourself to become a Paladin you'll have the highest defensive boost of any class in the entire game. You'll also have the second (or third) highest attack bonus in the game.
List of Dragoon Class Abilities:
(Level 1) Ply: Heals a Single target.
(Level 2) Blast: Hits 3 targets with 25 Power.
(Level 4) Thorn: Hits 3 targets with 35 Power.
(Level 5) Cure Poison: Cures Poison/Venom from 1 ally.
(Level 6) Avoid: Causes fewer monster encounters.
(Level 8) Wish: Heals your entire group for 80 HP.
(Level 10) Mad Blast: Hits 3 targets with 50 Power.
(Level 11) Cutting Edge: Hits a single target +32 Attack.
(Level 13) Restore: Removes Stun, Sleep, Delusion and Curse from a party member.
(Level 16) Ply Well: Heals a single target for 200 HP.
(Level 17) Briar: Hits 3 targets with 70 Power.
(Level 22) Wish Well: Heals every member of your group for 160 HP.
(Level 27) Fiery Blast: Hits 5 targets with 110 Power.
(Level 34) Pure Ply: Heals a single target for 1,000 HP.
(Level 36) Nettle: Hits 5 targets with 140 Power.
(Level 46) Pure Wish: Heals your entire party for 400 HP.
All in all the Dragoon class tree is extremely powerful if you've got a weak party or (for some reason) don't have your entire party available to you at the time. As with most games, Paladins aren't built to do the most insane damage out there. They're built to survive and that they do quite well.