If anyone is still struggling with Cecile's quest I hope this helps. This was done without spending any trait points and mostly using lona's starting equipment only. (Had to get rags due to 0 points in scu, I don't know how the other guy did it without)
I used a bomb to kill the guard
because I simply couldn't resist, but that's entirely optional. If you strike him from behind with the wooden club he will succumb to the power of the bonk.
If he looks like this he will be out for a minute, giving you enough time to talk to Cecile after which Vineet will reveal that he is somehow even sneakier than our little ghost. After the conversation Cecile and Vineet will immediately gang up on the guard, which should be an easy fight but I recommend helping out by setting the guard on fire with your lantern a couple times or attacking him from behind. Attacking with the club and setting enemies on fire with the lantern will inflict head- and chest wounds which lowers their defense allowing Cecile to finish them off very quickly.
It's important to note that the lantern dot stacks independently with itself, and each tick has the same chance of inflicting a wound as a strike with the club. This makes the lantern a lot more stamina efficient than the club, but you may have already noticed that it's much harder to successfully land a hit and unlike the wooden club it doesn't bonk, which you will need to keep Cecile from taking too much damage so make sure to position her opposite of Lona like this.
If you know what each weapon is good for and have this formation the rest should be easy. Just kill them one at a time while keeping an eye on your stamina. You don't really need it but you can do a lot of damage with the mines at the entrance if you want.