Seeing as the whole point of the spell is to be in a certain area to have it inflict damage, then throwing a firebomb is pretty darn effective because it either a) forces them to move out of the area, thus making their spell worthless if there are no enemies there anymore for the spell to affect, or b) forces a concentration check if the caster stays in the area
I'd also note that "entering the area" is the same as having you in the area when it explodes, especially if you are sitting in the burning area for a while before it gets to your turn. Same logic as pushing a target into an area spell when it isn't their turn. I.e., the affect isn't triggered when it's your turn and you intentionally move into said area--it is effective the first time your PC is exposed to the area.
Maybe this is a playstyle difference, but as far as I'm concerned the point of the spell is not to "be in a certain area," it is to "be within a certain distance of enemies." There's nothing that ties you to a specific area--it's a mobile spell. Even if you manage to fill up a solid 15' x 15' area with nine firebombs, so that the cleric has nowhere to go without getting roasted or moving more than 15' away from where his enemies are right now... there's nothing preventing him from moving where the enemies aren't, and then letting the other PCs follow. If the enemies had a good ranged capability you wouldn't be fighting at Spirit Guardians range anyway, so they pretty much have to either follow you or break contact. And if they were willing to break contact and flee, Spirit Guardians wouldn't be attractive in the first place. Add that all up and I just can't see more than maybe 10% of situations where flaming oil is actually helpful in countering Spirit Guardians. That 10% is composed of 1% pitched battles on a fixed defensive position, and 9% times when somebody can grapple the cleric and hold him in the flames.
I strongly disagree with your ruling on "entering the area." I think spells and effects which are phrased that way are phrased that way for a reason. I don't consider creating an area around a creature to be equivalent to "entering the area". If you rule that way, Moonlight becomes a mobile deathray doing double damage and Evard's Black Tentacles forces
two saves doing 6d6 + restraining before the target ever gets a chance to respond, at the cost of an action. Both of these spells become incredibly good under that interpretation, and I don't believe that's intended. So why would I apply a different interpretation to burning oil? Clearly burning is supposed to be something which takes a few seconds to burn you, so you have the chance to move out of it on your turn without getting burned.
Final point: if your ruling were applied to burning oil, no one would
ever throw it as an improvised weapon the way the PHB suggests, because it would be strictly inferior.
Given the way you're ruling burning oil I can see why you'd use it against Spiritual Guardians: it is, as you say, an auto-hit. But it doesn't work by PHB rules as I read them.
