The script for this chapter can be found here.
Subjective: The following part of this article is subjective to the editor's writing. Therefore, it may not work for everyone.
This chapter teaches the player about the attack stance whereby having two units side by side on adjacent tiles, grants the ability for both units to attack during an offensive and defensive move. It also teaches the player about the use of the Dragon Vein ability.
A good strategy to use for this particular chapter, due to the nature of the following chapters, is to maximize experience gain for your Avatar, rather than to either of your other two characters here. To achieve this, use the Dragon Vein ability to remove the debris to create a defensible area that also includes a healing zone. Then use Gunter and either Jakob or Felicia, to block and weaken enemy units enough so that using your character in a follow up attack, gains credit for the kill.
Note that Felicia and Jakob have a ranged attack that can reach enemy units without allowing for retaliation whilst Gunter can take a lot of damage before being defeated. Using both of these to your advantage would be of great benefit.
A good strategy for the higher difficulties such as Lunatic is to lure both groups, left and right, down to the square zone in the middle of the map without using the Dragon Vein. Once they reach the square, retreat back down the bottom left or right of the map, forcing one of the two groups to take longer to reach your units. With no clear route, one of the groups of enemy units will be forced to circumnavigate the entire square to reach your units. Gunter can be used to boost your Avatar's defense stats so as to be able to take less damage. With only half the enemy units within range, pick off each of them one at a time, being sure to use Jakob or Felicia on the boss of the chosen group to weaken their stats. Once they are dealt with, activate the Dragon Vein for healing in the zone and engage the final group inside the square.
As a final aside, it is worth mentioning that, though Gunter and Jakob/Felicia are both pre-promoted units, Jakob and Felicia both gain EXP as if they are unpromoted units. Killing enemies with Gunter will result in the loss of valuable experience, especially on Conquest, but feeding a few kills to Jakob or Felicia won't be a waste.