I think there are two ways to read the spell, none of which match yours.
1. - Each berry is it's own area attack, they are just triggered all at the same moment. So damage for each berry would be rolled sperately and fire resistance apply against each berry individually.
2. All berries are a single attack, but the placement of the berries alows you to shape the affected area as you desire. Since it's one attack, you only take damage once, even if you stand within the radius of two or more berries.
I tent dowards 1., but in either case, you don't roll 8d8 and reduce the total by the amount of fire resistance. Sou roll 1d8 eight times and apply fire resistance to each 1d8.
Ogoun the oracle used the spell "seeds of Fire" (Level 6 druid spells, p.282 CORE) in this way: He Created "Holly Berry Bomb" and inflicted 8 x 1d8 + level fire damage. Now, you think it is appropriate that the resistance to energy is applied to each individual berry? A monster with fire resistance 10, that is hit by an explosion of 8 berries will apply the resistance, once or 8 times (once per berry)?
Reading the rulebook I noticed that the resistance to energy applies to any "attack", so you might think that 1 = 1 attack spell and then, if Ogoun created 16 Berries with 2 spells, the resistance should be applied only twice. Is that correct?
What do you think?
Thank you!
For purposes of Invisibility: the total attack is one attack.
But normally, Per Berry is the attack. So if he rolls good, it is still a decent amount of damage in total.