Olivia

Olivia can be a powerhouse, wall, debuffer and dancer all at the same time if you place her well.
I've used her in almost every possible match for that simple reason; she fits in every team with this build. And also that her stats synergize extremely well with each other. Many builds instead use the Moonbow + Wo Dao+ tactic, but in this case Olivia is more of a "I'm able to take a hit and attack as well, but only if necessary." In case of being attacked by a melee fighter, she fires back hard because of her Silver Sword+, dealing reasonable damage to even blue units. This is also why she needs a +Attack boon. The most important part of this build however, lies within her defensive stats. Even though we're aiming for -Def bane, this is fixed by giving her Pavise. You'll be surprised to see how much damage she can take when she has Pavise. That's also why you'd want a -Def bane over a -HP bane. This build features Fury over any other A Passive, because it ups all of her stats except HP, this brings her Defense up to 28 and her Resistance 31. Resistance 31 makes Olivia a great candidate for Attack Ploy 1. Which preferably needs to be upgraded to 3 for maximum usage. Taking further advantage of Fury 3, in case you'd like to teleport to the rest of your team, swap out Wings of Mercy 3 for Escape Route 3. Fury 3 helps Olivia to quickly get to half/below half of her HP and you'll soon see her dance around the map.

Olivia is going to die to basically every blue unit anyway, so this build aims to make her more effective against green units. This allows her to act as a green check/counter in a pinch. Ruby Sword+ is chosen over her basic Silver Sword+ to give her triangle adept while freeing up her A slot for Fury. Fury is great because it gives her some important defensive boosts, and the chip damage doesn't matter because Olivia shouldn't fighting units most of the time anyway. Wings of Mercy is the obvious choice as a dancer, allowing her to save some low health units in a risky situation. The C passive is open to what your team needs, but Hone Atk and Hone Spd are solid options. Threaten skills are not as effective because she is better off staying near teammates and buffing them rather than staying among the enemy.

Being the first summon from my game, I've always treasured my Olivia and learning that she's +Atk made her even more valuable to me. I can send her into the front lines when required without having to worry.

IVs: +Spd -Res

Steady Breath is easily one of the more powerful skills of the game, and can make any character a defensive threat and wall simultaneously. Even dancers are not entirely exempt to this rule, and this build aims to take advantage of that, given Olivia's surprisingly decent defense of 27, on par with that of Ryoma.

Simply put, this build makes Olivia a duelist wall, giving her slight flexibility with how she can be positioned to dance and support allies, while serving as a powerful green check for teams. With steady breath and luna, Olivia is able to tank and finish off many greens during enemy phase, most notably Hector and Cherche, while being able to finish off others (such as B!Ike, Amelia, Axe!Azura) on player phase afterwards. Her combat capabilities with this setup go so far as to make her able to take on several threats such as fury Ryoma, and Lucina if they are not +spd. This makes her a powerful wall and check while maintaining the support utility that gives Olivia her uniqueness.

Wings of Mercy can be run as the normal dancer option, though if desired Escape Route can be used after facing and defeating major threats, though the risk in using this is generally unadvised given her low HP. Alternatively, if one wishes to further specialize Olivia as a fighter and forgo teleportation dance mechanics, swordbreaker or quick riposte can be run to secure kills more easily, particularly against faster threats. It should also be noted that Spd +3 seal can also be run to mitigate this, by putting her at a more comfortable 39 Spd with +Spd to avoid many more doubles.

If one does not wish to use a B!Ike for steady breath, steady stance from the free Black Knight from the Tempest Trials can be used as a substitute, but it should be known that this severely hinders her otherwise surprising combat capabilities, limiting her to a purely defensive role.

Build focused on making Olivia a strong a reliable dancer that can survive first contact with most enemy units and fend for herself when she is in a tough spot.