Titania and Soren

I've personally never made use of Titania, I've had much better Axe units since before I managed to get her and I've never made a Horse Emblem team. Though I do hear that she's fantastic choice for a Horse Emblem team who needs not only a green unit but an anti-mage unit, due to her high Resistance. Her awful attack stat and lack of a legendary weapon to offset it makes her quite weak in anything other then a mage tank.

I have seen LnD + Desperation Titanias as well though, and they end up with such a high speed that it kind of offsets the lower attack stat and her defenses are both on the higher side so she doesn't become "too" squishy with it. It's definitely an option.

As for Guard, it's not that great. There are many different B slots that are a thousand times better on basically everyone. I guess if you're building Titania herself as a tank/off-tank and don't have a better B slot to use then you can make use of it on her, but I wouldn't bother putting it on anyone else.

As for her stats, basically a boon in anything works for her, except HP maybe. One of her offensive stats would be the best options though, +atk to counteract her low atk stat, or +spd to push it over the top. As for a bane, they're all kind of rough. Def might be the best, since she's usually used as a mage-tank anyways, but it also makes a LnD build far riskier. If you do plan on going the offensive LnD build maybe go with a bane in Res if you can cause it's high enough that even as a bane you won't be completely nullifying the stat. I guess it depends on if you want her to specialize or be more well rounded.

Soren on the other hand was one of my main Green Mages for a while, mainly cause I didn't want to be the hundred thousandth person to use Nino and only Nino lol

I tried two builds on him that I liked best. The first being the typical "get the green blade tome and use LnD to just crush people" build. The Nino build, basically. Desperation would also be your best B slot for that, obviously lol

What I preferred though was the Raven + triangle adept build. His stats are well rounded enough to go toe to toe with most mages just fine, but give him this build and he can also crush most colorless units.

As for stats, a boon in atk, spd, or res works well, and bane in def or hp would be your best bet.

EDIT: You can use Watersweep on anyone with a decent spd stat who you want to be safe while attacking mage units, so it usually works best on another ranged unit. Basically if you don't think your unit will be able to finish off a mage unit in one round and you fear the returning attack might finish off your unit instead, then just go with Watersweep and slowly chip away at them since they can't counterattack on your turn.

Not really a meta build, since a lot of the meta mages will just blow you up on their own turn with how offensively focused they all are, but still a viable build on a team with another mage with a high Res who can take the damage and bait out enemy mages.