I picked up my first blue quality gear during the Nighttime solo Tortage quests.
I got a High Priest's Necklace, a Robe of Apprenticeship and for some reason didn't get a blue weapon after killed Strom (I thought all classes got one here, ho hum).
By now there is only one spell worth casting - the feated Fires of Gehenna. Other spells do damage in different patterns but the Fires spell is so powerful and if you're fighting 3 mobs it's more useful to get one down fast than to spread damage evenly. Backing up a little will often manoeuvre mobs into position where they get splashed, as will switching target to the middle mob.
Fires does 175-195 Fire damage to a level 20 mob with an Incinerate dot that does 1-2 damage per stack and stacks up to 5. This means you kill them pretty consistently in 5 nukes.
Deeper Fires 5/5 +5% damage on Fires of Gehenna.
Fluidic Fires 5/5 Improves Fires of Gehenna with a splash effect that damages enemies near the primary target. +100% splash effect.
This is rather misleading. First Fires starts with a splash effect for about 50% unfeated. Also the effect doesn't become 100%: the main target is taking 180-215 the secondary target takes 119-135 (based on quite a small sample).
It definitely does more splash damage though as you develop it with Feat points.
It's clearly worse than the other option - straight fire damage - but I wanted to investigate it for this blog. It illustrates that Funcom's documentation on the tooltips still doesn't match the in-game effects even 2 years after release.
Amassed Flames. This is a + fire damage buff that quickly builds up 5 stacks and stays there as long as you're chain nuking (one per cast of Fires). The stack drops off completely if you don't cast Fires for 6 seconds. Damage goes up as you build the stack: 4.2, 8.5, 12.7, 17, 21.2 fire damage.