Eternal Guardian

You don’t actually need Rhea in your party since she’s been missing after the timeskip event in this route. However, this battle takes place inside the Holy Tomb, with Byleth as a critical unit in this battle so keep him/her safe to avoid failing the mission. You’ll start off from the south and you’ll have to defeat all enemy commanders to clear this Map.

There are three giant golems roaming the main hall. Like demonic beasts and other giant enemies, they have shields that you can destroy in order to be able to deal normal Damage. This is pretty much a requirement since you can’t deal magical damage to it until you destroy the barrier. The guardians also have really longer Attack Range so expect to get their aggro from afar.

From the starting point, divide your units into two groups to cover both sides. Start off by defeating the nearby enemies on both flanks, then going downstairs. To avoid overextending, have some units with high defense or evasion to deliberately enter the enemies’ Attack Range and lure them to your waiting forces.

Move your forces together so they can easily Support each other. Deal with the first golem, preferably have your mages use Gambits first since they can’t Damage the guardians with the shields up anyway.

After taking out the first guardian, carefully proceed to the north, preferably while staying near the walls so you can avoid getting flanked yourself. The challenge in this part are the enemy mages that can use Bolting, a spell with an exceptional Range, which will prevent your units from retaliating. If you have a Pegasus rider or Falcon Knight, they’re great counters to these mages thanks to their high movement range and high resistance. If not, you’ll have to take them out quickly preemptively using your snipers or other high-mobility units.

Once the mages are taken care of, you can now proceed to aggro the remaining guardians and slowly move your units to attack the enemy forces in the middle of the Map. The platform in the middle can also serve as a divider between the two guardians so it’s even possible to wipe out the enemies in the middle and just deal with one guardian at a time.

After wiping out the guardians, you can now surround the remaining commanders. They won’t budge from their positions so you can gather your forces outside their Attack Range first and come crashing down on them, all at once, to wipe them out.