|This unconventional Soliel build is aimed at throwing off any opponents who challenge you in arena, as well as being an effective physical tank that can be effective in the player phase as well as the enemy phase with the right team synergy such as a defensive or offensive support buffer. Soliel has an incredible stat spread, especially with neutral stats, she has the best mix of attack and bulk out of nearly every sword unit in the game. This is why neutral stats are preferred.
Allies: The ally of choice for this build would be New Year Azura. She can run Drive Def 2 in her c and seal slots to grant all allies a defensive boost within two spaces with the added bonus of having a similar effect in her weapon Hagoita+. Another good teammate to run with her on this build is Vanguard Ike with Def Tactic 3. As this will increase her defense with a flexible range, allowing for a variety of team formations. Earth blessing is best in this case for the extra ATK and HP in the earth season.
Weapon: Her weapon of choice is Firesweep S+ as it allows for Soliel to strike back during the player phase without fear of being attacked to increase her survivability, which is useful in arena defense. However, while she can't counter attack on the enemy phase, her special helps out with that. If you want the build to be slightly more flexible, then Silver Sword+ is preferred as she can strike back during enemy phase to allow for quicker special charge while retaining the same attack as with the Firesweep S+, with the added bonus of refinement. In this case, it is better to take either the ATK refinement or the DEF refinement. The refinement will also help increase her HP which is slightly lacking.
Assist: For her Assist skill Swap is preferred as it allows for tight and more co-ordinated movement between team members, something that is crucial to this build as she prefers to have supportive buffs from her team to increase her destructiveness and tankiness. Other team members should be running Reposition OR Sing / Dance for greater movement options among the team.
Special: Her only preferred special is Pavise so that her tankiness is ensured. An ally running Infantry Pulse 3 will give her special an instant charge. (Be mindful of the HP increase from refinements and blessings)
Passive A: Depending on the weapon choice her preferred passive should be Close Def 3 as the main utility of this build is Soliel's frontline physical tankiness. This passive is more synergistic with the Silver Sword+ as it grants her more defense than steady breath and there is no need to make the cooldown faster. With Firesweep S+ her preferred passive is Steady Breath because she can't counterattack. Meaning that she will need her special more often as the fight will be more drawn out.
Passive B: The absolute best B Passive for this build is Shield Pulse 3 because it grants her a special cool down of -2 and reduces damage when her special triggers by 5 to increase tankiness. No other passive synergises with this build so well.
Passive C: This passive is flexible, but her default passive, Drive Res 2 and Threaten Atk 3 are preferred depending on how you wish to support your allies with this build.
Sacred Seal: Finally, her sacred seal has two options. You can either run Close Def 3 for that extra bulk or you can run the more situational but more effective Deflect Melee 3 to act as a second, more powerful Pavise for when the opponent doubles Soliel. (Such as against an Ayra).
(Side note: This untested so i don't know if it works, but if Pavise activates on the opponent's second hit with deflect melee, the effects could stack and block at least 100% of the damage providing that it is an adjacent attack. Probably not how it works though.)