+Def -Res Soleil?

May want something like +Atk or +Spd, but you can always merge the +10 on a new Soleil with the correct IVs. I've got a Soleil with DC and she does wonders. Soleil would be better for Arena because you can +10 her easily. Ike would be better if +10, but a big part of that is because he's got a blessing.

If you're going to go +10, you can work with most IVs for now, and +def -res is definitely fine. Makes her a decent duelist, but you would probably need a healer or some build which is slightly less suitable (still fine) with -res or -HP and +atk or +spd. For example, with +def -res I tend to go for something like wo dao+spd, brazen atk/def, and spd smoke and then either wrath noontime (or sol if using flashing blade seal) or moonbow + renewal, which takes advantage of her now very decent def to take counterattacks, but with +spd -res I would either play firesweep L&D galeforce or some swift sparrow desperation build.

Soleil+10 I would expect to be generally better than L.Ike+2 with any IVs on either side because she hits harder and faster. In PvE it would make a significant difference because getting doubled a lot is generally going to get you killed unless your defenses go at least into the upper 30s. The two advantages that Ike has are:
- he gives earth blessing bonuses, so for arena scoring if your team uses earth blessings and it's earth season he's actually as good as +10 Soleil even when he only has something like...4 merges? I haven't tried this out on the calculator.
- consistent one-round DC aether even if he doesn't get doubled. There's simply no way Soleil can pull this off. She could use slaying edge+breath skill, but then she wouldn't have DC, or if she uses DC she can't use a breath skill. The closest thing she has is wo dao + wrath + noontime (or flashing blade sol), which can work due to her high atk stat, but she has to use up her B-slot for it and it ties her to the wo dao.

In general, I consider DC to be a very specific play on Soleil (and if you don't know what you're doing it's just mediocre or even bad), especially if she doesn't have summoner support. It doesn't make much sense to me to drop her res when she's using DC, so you're probably dumping HP, in which case she has 44 with a weapon refine at max merge, which is definitely on the low end for a +10 melee unit, which means she can't necessarily survive more than one hit regardless of which defensive stat gets targeted. And if units can't survive multiple hits, then I find it hard to justify DC unless you have something insanely explosive like Ayra (who also has just enough def to maybe survive a second hit to her def) or you can heal every single round back up to a point where you'll be able to take another hit, which is what I designed with wo dao + wrath + noontime.