Subjective: The following part of this article is based upon the editor's personal experiences and opinions, and therefore may not be applicable for all readers.
This is your final battle. If you've been saving stat boosters and Legendary Weapons for a rainy day, this is the time to bring them out. Make sure to replenish/equip all battalions before entering the battle, as well as all weapons.
Rhea has torched Fhirdiad, creating tons of fiery floors which halve movement of all units as well as damage them every turn; alongside stairs, for this reason Cavalry is strongly discouraged. In addition due to the fact that fire drains and slows down units, bringing several units with healing items, Heal, Recover, Physic, and Fortify (Only available on Mercedes who you must have recruited pre-timeskip). Warp is also incredibly useful for warping units and saving time walking to places. In a similar vein, Flying Units are incredibly useful since they are immune to fire and can ignore the effect of stairs.
Your units are divided into sections, with 6 in the center, with the other 6 on the bottom right with a path that leads up to Catherine. Your first order of business will be to group of your units back into the center of the map, but before you that, you need to dispose of the Paladins and Snipers at the beginning. For the bottom right, you need to take out the Altered Golem that stands right in front of your unit. Their 1-4 range in conjunction of their minor Crest of Seiros, which grants them higher damage output, makes them formidable foes. What's worse, every three turns, Rhea will roar granting permanent stat boosts until +10, making them outright lethal. Luckily, most of them won't attack until provoked, so only the ones on the way to Rhea have to be dealt with.