Team Requirements:
Sword/Lance/Water Damage: Matthew/Elwin/Lewin/Bernhardt are wonderful. Pierre is a decent water unit as he can keep himself up with Water Control and climb the mountain in a turn from outside max range but Lewin/Matthew are much better choices. Horror Helmet is a great SR helmet choice because of the 20% defense reduction proc.
Debuffer/Dispeller: Elwin and Lewin both have Roar which dispels both the ATK and INT portions of the dragon's buff. Bernhardt has Hegemony but there is a chance you get the defense buff instead of the offensive one so be careful and ready for that when Thunder hits.
Elwin also has the added benefit of being able to disable passive skills with Detect so you can still lose a unit and not wipe.
Healer: Almeda is great for this, She can debuff from 4 range at 4* and passively heal alternate turns. Any healer will do though.
Tank: Grenier is good because of the faction buff and passive guard. Any cover tank can be used but they won't be doing damage. Last Knight is a good option to put on your tank as ranged attacks can proc the ATK debuff.
Trash Slayer: Cherie or Cavalry unit. Fliers are good because they can swoop in for two hits and then back out before Thunder, too. Mounted units are better for killing the archers, but Cherie should be able to use Wild Princess and kill both.
Recommended team without needing pulls:
Almeda - 4*
Grenier - Royal Vanguard
Matthew - Hero focused on damage and self sustain
Lewin - Swordsman with Roar
Cherie - Flying/Trash clearer
Thunder Dragon Map Enemies at 45.
2x Level 45 Water Generals with Tide Masters. Abilities: Last Stand and Water Control
1x Cyclops. Abilities: ATK Intimidate and Roar
2x Dark Elves. Abilities: Decay and ATK Break
1x Thunder Dragon. Abilities: Blitz Drake. [Passive] Before attacking with a ranged attack, damage taken in the ensuing battle reduced by 90%. [Passive] Can act/attack again after eliminating a target.
Blitz Strike: 2-turn cooldown, 4 range, single target: [Physical Damage] Deals 4x Physical Damage to target. After battle, ATK and DEF increase by 2 turns. Also targets hero directly.
Thunder: 2-turn cooldown, 3 range from dragon, all targets: [Physical Damage] 3 blocks around the caster, 40% chance to inflict the [Electrified] effect.
Thunderfall: No cooldown. Deals 5x Physical Damage to target and 50% chance to inflict Electrified. Global range. (Never used)
Electrified - Damage Taken +30%, Attack -30%. Heal after this if it hits your dispeller.
Long version of clear.
The trash monsters are pretty straight forward. Lure out the water units your first turn with a unit and your tank guarding all your units. They do have water control so the tank will need to be in range of your healers if they can be reached. The Cyclops can be lured by your trash killing unit or your sword fighter which will also bring the archers closer. Kill the Cyclops and destroy the archers before setting up for the dragon on the mountain. The archers will prefer to use Decay first before a regular attack so remember to heal the decay away before using anything valuable like a faction buff.
Thunder Dragon has a very distinct attack pattern at 45 of Blitz Strike, Thunder, Regular Attack, Blitz Strike, Thunder, Repeat. The general strategy is to debuff the dragon after Blitz Strike with a unit that has Roar while keeping your other units out.
By turn 5 you should have all of your units at 5 range which is one space out of the danger zone. On turn 6, move closer with your dispeller and attack with your water unit/trash clearing unit. (Only move your flying unit in IF they can survive a hit from the dragon, otherwise they will attack them over your healer and get a kill. A second action and he will acquire a crazy offensive buff.) Position your Almeda/Healer 4 blocks away from the dragon. Three blocks down and one block to the right is best while keeping your cover tank right below her. She is able to debuff the dragon from here and provide heals to units on turns 3,4,6,7,9,10 of the dragon's attack phase by moving forward one space along with her cover tank. If you are on a turn where Thunder isn't coming you can move her up one with her cover tank behind her and heal the units on the dragon.
Bring everyone to attack for two turns and retreat after it uses Blitz Strike again to at least 4 blocks away before it casts Thunder on the following turn. Keep your dispel unit inside the 3-block range and have him eat the Thunder cast alone.
TLDR Version
1. At turn 5, you should be at max range with trash cleared and on turn 6 move units in. [img1]
2. Bait the single target attack first with your healer at 4 range in the woods while being covered by your tank. Move in the dispeller swordsman after that attack and remove attack/defense buff. [img2]
2. Keep units at 4-block range for 3-range Thunder.
3. Move healer in 1 space to heal with tank behind them and attack with other units. Dragon will regular attack and then Blitz Strike to buff up again. Dispel the buff and back up to 4-block range with your other units. [img 3]
4. Rinse and repeat.