The ideal strategy is to use both Elwin and Lewin if you have them, otherwise, if you're missing one or the other, it'll be hard but still accomplishable. Ideally you would have to be level 45 (assuming that you haven't beaten it before) and have decent equipment on your units. Optimal units to use are:
Elwin/Lewin If you have both, use them as well,
A faction buffer for Lewin (i.e. Dieharte or Freya),
A healer with dispels for debuffs (ideally Tiaris, Liana, or Almeda),
and either a 2nd/3rd DPS or 2nd Tank
My lineup was :
Elwin
Lewin
Freya
Tiaris
Ledin
Ideal Equipment loadout would be anything that provides ATK + DEF for Elwin and Lewin. Pure DEF loadout for the tanks and bonus healing or bonus INT for the healer.
The minor mobs should be easy enough for you to slowly clear your way up to the dragon, but make sure you keep your CDs in check in order to get through properly. At this point, before engaging the dragon, you should rebuff and heal up if necessary. Have your tank(s) walk in first to draw aggro and to lure out the thunder dragon's AOE attack so that your DPS can follow up with damage without worrying about getting wiped. On your next turn, use your tank's guard ability to protect the DPS from further damage. Once your DPS has finally arrived in front of the dragon, make sure they're ready to dispel any buffs that the dragon has on them currently (ideally removing the bonus ATK%) and use your self-healing skill (Sun Slash, etc.) when it is up and Roar to continuously debuff the dragon. Keep your healer out of range and have them continuously keep the DPS's HP % above 80%. If your tanks fall while the dragon is still around 50% HP you should retreat and upgrade your troops and gear more to be able to survive. If they fall while the dragon is around 25% HP, don't worry! You'll still be fine, just keep doing what you've been doing up to this point.
Further personal loadout for those interested in what I used to fight the level 45 Thunder Dragon:
Elwin 50 (3*) - Wind Cutter Dagger (20)
Mithril Armor (20) Horror Helmet (20)
Esoteric Slate (20)
Full Full Moon enchant to increase DMG output
Lewin 50 (6*) -
Injunction Sword (40)
Mithril Armor (20)
Chain Helmet (20)
Judge Talisman (20)
Full Clock Enchant to reduce CD on Roar
Ledin 54 (5*) -
Oath Sword (40)
Aeneas' Armor (20)
Aeneas' Helmet (20)
King Amulet (20)
2 Ice enchant + 2 Hard Rock enchant to maximize DEF and MDEF
Freya 50 (4*) -
Blood Lance (20)
Holy Steel Armor (20)
Berserker Helmet (20)
Greaves (10)
2 Ice 2 Hard rock for max DEF and MDEF
Tiaris 54 (3*) -
Blessing Staff (30)
Ghost Robe (20)
Sage Hat (20)
Crystal Ball (40) Full Crystal set to increase Healing Potential
Skills Layout -
Elwin - Detect, Sword Soul, Sun Slash
Lewin - Roar, Detect, Battle Cry
Ledin - Divine Guard, Light Reflect, Vindication
Freya - Barb, Lance Phalanx, Power of Hope
Tiaris - Heal, Mass Heal, Miracle
(Yes Frey's barbs don't work on Dragons, but that's not why I brought her. They still work on the normal mobs to reduce clear time and she's still a damage sponge in the end, so Lewin and Elwin can do their jobs.)