Flamers are a great offensive unit, but they are toast as soon as the enemy gets a chance to shoot at them, and they will because they are way too dangerous to let get close. Because of this the best way to use flamers it to plan to deepstrike, toast something, and then die. Just about any other strategy involving them just goes straight to the dying part.
Fateweaver seems to be a lot of points to spend on one unit, epecially since you also have 2 heralds. I would suggest dropping him and both heralds with blue scribs. Beyond that i have no comment as that is the extent of my knowledge on the subject (need to spend more time with the codex and play more games against my daemon buddies)