Yeah, just killed the guardian by having it teleport into a circle of 6 fully buffed clerics who meleed him to death in under 2 rounds. Honestly don't think the spell-boosting feats are any good. Flame strike is pretty much the only spell it helps and it's so slow to cast + not a great AoE so it's somewhat hard to aim well and generally not worth the time. The only instance I've really been using it is when I sleep whole groups (e.g. the hook horror ambushes before underdark) with Greater Command in an area, then have my whole group of 6 clerics unload 6 flame strikes at once. Granted I could probably do that a lot more than I am now, but it's more fun to use physical attacks.
You're right about the Elemental Feats being overrated, both for cleric and wizards. The cleric's main advantages are buffs or mass disables (eg. Greater Command/Symbol of Hopelessness), neither of which benefit from the elemental feats. Even getting it on wizards is rather dubious, since 20% damage on a 1d6 fireball capped at 10d6 isn't that big a deal, especially since Skull Trap is superior.
There is one exception, and that's if you have a Cleric of Lathander. While Flame strike is pretty weak, Clerics of Lathander get Fire Storm as a Level 6 domain spell, which is otherwise obtained by clerics as a Level 8 spell. Fire Storm hits like a freight train, so an extra 20% damage is always nice. I'd still rather get Spell Focus: Enchantment and Weapon Focus though.