The teenage Crossbell patriot with police academy experience enters Thors Military Academy as a member of New Class VII for Trails of Cold Steel III and IV. She comes with the Skuld earth-elemental Master Quartz that increases break damage while HP is high (and later gains regenerative ability) and Brave Order Sledgehammer which significantly increases the party's break damage for four turns. Juna also has two stances with her weapons for two styles of fighting: Striker Mode improves her physical strength and defense and deals stronger blows to single targets, while her Gunner Mode will enable her to cover a range from further away at the expense of damage and hardiness.
Juna is in a very unique position of having four lines in her ARCUS II with an impressive weapon range. Two of the ARCUS II slots are locked into Space elemental usage only, while one has to be earth elemental. Her base stats offer impressive survivability and respectable physical damage, and while the arts defense leaves something to be desired without augmentation from accompanying gear or quartz, she can be staged away from clustered allies and provide damage in Gunner Mode or support as well to avoid AoE attacks that run off enemies' art attack.
Her crafts have a great deal of situational utility:
Cross Break guarantees to impede if it connects, but is melee range and, in Cold Steel III, has poor reach and in most situations would only hit one target. In Cold Steel IV, when upgraded, it carries a good chance of causing the enemy to faint. This attack deals very high break damage.
Gemini Blast is a low-power AoE move with an enormous range that carries a chance of reducing targets' physical defense and speed. The amount reduced improves when the craft is upgraded.
Brave Smash has great odds of unbalancing whatever Juna hits in a straight-forward tackle. This also carries a good chance to cause seal status on enemies, leaving foes with no arts powerless to fight back. This is great for raw damage.
Quick Star moves the turn order of allies other than Juna to immediately follow her use of this craft, which can be a game-saving maneuver. Additionally, it grants a speed buff. Exploitable when grouped and used with Machias's equivalent skill.
Burning Heart is only available in Cold Steel IV, and boosts Juna's strength and defense while providing the team a Brave Point.
Build 1: AoE Debuffer
Pink Lloyd - Comfortably Numb
Because of Juna's ARCUS II setup and the gigantic range of Gemini Blast, she is the only character available through most of CS3 and CS4 that can pull off disabling entire squads of foes near the start of the encounter. Four (one interchangable) status effects are at the core of this concept - Seal, Mute/Freeze, Petrify, and Faint - and focus should be given to improve these quartz to get these conditions the highest available efficacy at any point in the game. As many as all of these statuses can proc per strike. Speed must also be stacked so she can pre-emptively disable combatants. This build best works with physical attackers equipped with Thor alongside Juna, as it gives a tremendous damage output boost to all of them.
Line 1:
Suzaku/Seal III
Nohval Gem/Action 3
[Sevenfold Gem/Impede 3]
Goldia Gem/Hit 3/Bluster/Carnelia Gem*
Line 2:
[Genbu/Petrify III]
Line 3:
Seiryuu/Mute III/Freeze III
Line 4:
[Faint III]
The Master Quartz given to Juna to make this build work are not that important, but I personally prefer Zephyr for its granted speed as the main quartz and Thor as a sub-quartz in CS4 so that she can take full advantage of the setup herself. If Altina is not a mainstay of the team, then Brigid or a Gladiator accessory is recommended to keep Juna's Craft Points high enough to spam Gemini Blast. This build is not particular about who should be linked with Juna for effectiveness, but Juna does have a tendency to guard allies, so Master Quartz and an accessory that improves physical defense further is desirable. Seal shuts down Juna, so Deep Ocher or Coral Charm are important. If they cannot be obtained, Coral Bracelet, Yellow Emblem, and Yellow Pendulum are good substitutions to keep Juna active.
What is great about status effect quartz is that they also provide good strength bonuses, so Juna can deal respectable damage even in the event that the opposition has great resistance or immunity to the status affliction.
* If Bluster is desired for longer-lasting status effects or Carnelia Gem is wanted for more raw damage, dual Coral Charms, Evergreen, and Rider Goggles are way to reach a 100% accuracy boost, and the former gives good strength.