Here's my Sebille build I used for two playthroughs. Absolutely dominated on Tactician eventually in mid-late game.
Talents: Don't get executioner on Rogue. That is best fit on Ranger who can actually hit multiple enemies easily on his turn. The best are Opportunist, Pawn, Parry Master(later in the game, if you decided on Dual wielding), possibly Glass cannon but I didn't really need it on her because of Flesh sac+haste and I'd rather her not get CC'd. The rest are up to you(Mneumonic, bigger and better, All skilled up are best)
If you're going to go Warfare and phoenix dive like ppl say that's fine. But you can afford to get a point into Huntsman to get Tactical retreat, which is better. It gives you haste for one turn which is great, and you don't have to burn yourself on fire for no reason. I used that on all my characters.
Pyro: Get Peace of mind. Must have on pretty much any character.
Aero: Teleportation is great. That's all you need, and even that you could do without honestly, if other characters have it. You really only need this if you plan on doing some multi-enemy backstab crippling blow combo.
Scoundrel: Up to lvl 5 for Mortal Strike or the 5-backstab source skill(I could one shot so many guys with this it was so fun, not the greatest but fun). Gives you crit multiplier(you should only be backstabbing anyway, so it's never wasted.). Adrenaline is a must have. Pretty much all the skills here are necessary or good to play with, except some Source skills and things like Chloroform(since you'll be going after low physical ppl anyway). Drop Throwing Knife later.
Warfare: Definitely max. Only skills I used though were Stomp, crippling blow, and whirlwind(never used them all together). I played around with them and they had their uses. But if you're not pairing Sebille with Death wish(great skill, use in pre-fight setup to power up your preferred damage for that fight), drop Whirlwind. It's not as strong as other skills you can use, since you'll be mostly focusing 1 target.
Dual wielding: I would max it. You get the dmg and extra dodge. Since Sebille is up close and personal it's pretty sweet later when you get like 40% dodge chance. Can dodge anything physical.
Polymorph: Chicken and Cloak, and maybe whatever else you might like. Level up alongside warfare, so you get points for Finesse too. Skills are going to scale off finesse.
Ability points: Only points into Finesse and Wits, Finesse priority. Crit chance is useless since you always crit on backstab, and you should always backstab. That said, Wits is great and you definitely could do with high initiative. You guys should try buffing before a fight so you don't have to waste turns doing that or worrying about Rogue going first and not being buffed. Memory when you need it, but you'll probably just want mneumonic Talent to help with that. Remember you get a free memory slot every other level.
