Update: I upgraded flashing blade 3 and gave her Aether.
Heavy blade is another good choice, as Soleil's 64 attack is higher than her 47 speed, but in the meta full of slow but powerful armour units and dragons, Soleil will be much more likely to outspeed these strong foes than out-attack them without an attack boon, Soliel's attack is beaten out by effie, a very common +10 in the arena, and also a very difficult unit for Soleil to counter. This skill makes use of Soleil's speed to proc her special more often.
I don't have the sacred coins to fully upgrade this yet. Odd attack wave and Even speed wave are both pretty good choices for Soleil, but I opted for odd attack wave as 53 speed is often overkill, and even though it will ensure flashing blade, the extra damage will boost Soleil's killing power and her special damage, but will also allow her great synergy with blade tome users, as an attack buff effectively adds 12 damage rather than a speed buff's 6. This not only allows this one unit to buff in any team, but makes Soleil a much more tanky unit in most situations. +6 buffs to both defences gets Soleil to 47 defense and 36 resistance, simply by repositioning or being repositioned. qui t'a t donne sous le Soleil 3 Et mme quand le fou se met en chepour. I unfortunately had to sacrifice a bridal Sanaki for this skill, but it has proved worth it thus far. This is where the build gets interesting. With Soleil's high speed and 36 resistance after a reposition, she can tank most mages and retaliate with a fatal blow. The premium A skill, worth lots of points in the arena, but also incredibly useful. For those tough blues that she normally would struggle against, a glimmer effect while removing the colour disadvantage, immediately followed by another attack can allow Soleil to take them out anyway. Galeforce and aether are definitely good specials for this build, but I went with Soleil's base special, because I didn't have aether fodder because the AOE specials act like desperation in that you get two consecutive attacks. I've been trying to get this weapon for ages, as the +7 defense is a considerable buff to Soleil's already pretty good defense, and when stacked with her B slot effect, she can reach 47 defense. HP is always a decent bane, as it leaves her good bulk intact, without sacrificing attack. It is unlikely Soleil will be doubled with any build, being 39 base at +10, but more speed grants doubles against 40+ speed units, and allows her to kill a large portion of the cast in one round when initiating.
She is a tank, utility and offensive unit all in one. I've been working on this building for quite some time, and I'm extremely proud of it.