Hey! Yeah, chapter 5 is a little more of a puzzle map, so it can be a bit tricky. A spoiler-free hint for it would be to look carefully at your units new Skills and abilities and see how and when they might counter the properties/weapons of the enemy units.
If you want a more spoilery answer...
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(Hopefully this is enough to get the "Read More" dropdown. Sure would be nice to have spoiler text on here...)
The gimmick of the map is that you're kinda escorting your escort unit. You'll want to place yourself/Luna near the Ally Guard on any turn where he'll be attacked to take advantage of the Moon's Favor Skill negating crits on adjacent units. You'll also want to find opportunities to heal him with Sarah where you can, especially if she has the Broken Artifact as its healing is dramatically weaker. This becomes a game of anticipatory positioning, choosing when to prioritize healing and when to skip it so you can utilize Moon's Favor as you'll often only be able to do one at a time, all while keeping the Guard between you and the enemy for safety.
If you want to make the map even easier (and net some extra exp for Sarah) you can block the Guard's path (by waiting at stairs or doors) to stop him in place and spend extra turns fully healing him. This isn't necessary to complete the map, but it can help if there's a particular section giving you trouble.
Additionally, there is one last trick to the stage where the Guard will be flanked by an additional enemy entering the map. This will require you to place yourself between the Guard and Sarah, allowing them to both take a hit for you while protected by Moon's Favor to ensure everyone survives the turn.