A battle against the turn limit!
King vs. Dragon!
This is a real tough one! I beat lvl 45 thunder dragon with Ledin lvl 52, Tiaris lvl 47, Matthew lvl 49, Almeda lvl 47, and Jessica lvl 35.
Defeating the dragon took 15 player phases, and 14 enemy phases. Losing a unit was not an option - everyone had a role to play. Do your best to efficiently move closer to the dragon while clearing out all the enemy units. You should be near the dragon's range by turn 5-7.
When preparing to fight the dragon, make sure all debuffs are healed off and apply any buffs right before entering the dragon's range. The dragon's AOE is beyond strong, one-shotting any unit that didn't have class advantage and high stats. So it was a 1v1 against the dragon. Teleport your main battle unit next to the dragon to save a turn getting hit while also saving a turn getting closer. Keep all other units just outside the border of attack range.
During battle you'll be relying on Healing Light, King's Will, Scions of Light, Divine Guard, Ardent Sun, Teleport, and Almeda's passive healing.
Always make sure "King's Will" can be activated every turn - the dragon can attack your hero directly, ignoring soldiers. Healing light should take care of the direct attack. If not, then you should use Ardent Sun. Make sure Ledin can always do maximum damage with Divine Guard. While healing light is active, attack the dragon, and Healing Light should go off most of the time, but don't if you don't want to risk taking too much damage. Don't attack when Healing Light isn't applied. When your strongest buffs run out or Ledin is low on health, have one unit go in and apply their best buffs or healing and immediately teleport them out of range and continue the fight.
With good understanding of when to attack and how to help your main unit, the dragon should be defeated within time.
Alternative Units
Don't have Tiaris? Try Lana! "Act Again" can be used to have a unit come in and out of range of the dragon to help your main unit. Don't have Lana? Try a healer with Regenerate to help in battle.
Don't have Ledin? Have your strongest sword or lance unit with self sustain, such as Lewin or Bernhardt instead.
Don't have Matthew, Jessica or Almeda? Yes you do, don't lie.
Hope this helped you defeat the level 45 Thunder Dragon!
Press onward, Commander!!!