Well, if you're not going to go all the way with Fire 9, but just other level 4 blesses, to go with W9N4, I suggest Fire 4. You have multiple attacks to start with, more with the quickness, and you want them to land even on shielded troops, so that +2 to attack really helps.
I can't remember the details of the Machakan spiders, but in general I reckon the tactic with a bless is to increase things the unit is already good at. So a unit with defense 10 doesn't benefit that much from a W9 bless compared to a unit with defense 16 (which becomes nigh-on invincible).
I do remember thinking that an E9 bless was good with the spiders, because then they get crazy protection. But I didn't think about it for long, so don't trust me on that.