Prologue Chapter 1: The White BirdMinor spoilers
Have Hong Hua weaken enemies for Eshmel to finish. Leave armored enemies to Hong Hua, and clear the cross-shaped reinforcements with Purifying Flash.
In the main battle, use 
Hong Hua to weaken enemies and 
Eshmel to finish them, giving Eshmel the experience. Leave armored enemies Eshmel cannot damage to Hong Hua. When he finishes an enemy, keep him outside the attack range of the armored enemy behind it. Once the reinforcements form a cross, hit all five with Purifying Flash. Check its 11 HP cost before using it. You must defeat every enemy. You lose if either of your two units dies.
Retreat to safety before using a Vulnerary
Turn on the full danger range at the start of training. Before using a Vulnerary, retreat to a tile enemies cannot reach. Enemies can attack farther away, so check the danger range. In the grass tutorial, enter grass before attacking; its Avoid bonus is 20%.
Use Tempest Spear against the tough enemy at the end of training. It costs 2 durability and adds 4 Might and 10 Hit. You still need to keep using Tempest Spear after the first exchange to finish the enemy.
Keep Eshmel’s finishing blow clear of the rear armor
Open with Hong Hua's Bolganone to weaken an enemy, then have Eshmel finish it with a normal attack for experience. If an armored enemy takes no meaningful damage from Eshmel, let Hong Hua kill it instead of insisting on feeding him the kill.
Continue advancing with Hong Hua setting up kills and Eshmel finishing them. Before Eshmel attacks, check his destination tile. Stop outside the rear armored enemy's reach so it cannot hit him after his finishing blow.
Clear the cross-shaped reinforcements with Purifying Flash
After reinforcements arrive and Blaze Arts unlock, use Purifying Flash to attack the five enemies in its cross-shaped area. One use here costs 11 HP and defeats all five targets. When later enemies line up in a cross, choose your target using the same area.
Leave one armored enemy for Hong Hua's level-up
Near the end, let Hong Hua kill an armored enemy to gain a level. After the Blaze enhancement tutorial appears, have her wait while Eshmel continues clearing enemies.
Eshmel
Hong Hua