Shiva's awakening is good, after it she pretty much replaces Asherah in most teams.
1st ability gets damage and cap up.
2nd ability gets from 20% assault to 30% assault, 20% water ele and crit (25% damage, about 30% chance).
3rd ability get cd reduced to 5t, damage up and 20 BG to self.
On burst effect: ability power up (damage and cap) for 3 turns.
Assist: When receiving damage small chance for -1T cd for skills.
And Hastur... I have Hastur and damn I hate her. Decent nuke - ok. Debuff... 8T cd and mediocre hit rate. Assault buff - eh, passable. The only great thing about her is DEF debuff and if it doesn't hit you have to wait several years till you can use it again.
But as for 15F, you can always use debuffs on 2nd stage of 15F, but I don't know whether with your wind just FBing it wouldn't be better, your team is faster than mine after all. Also do remember that SR have less damage with burst and Hastur's nuke and buff can be still pretty useful (damn I still hate Hastur, mainly because of her debuff skill though).