set up some hatches connected to a lever over the elves, and dump your oil filled barrels on top of them. create a 'wick' made of a dumped log on top of a grate. Pour magma over the log so it lights on fire, and wait for the fire to spread to the barrels. Pull the lever to drop the flaming barrels of oil on the elves.
Note that I'm not 100% sure this will work - chances are the magma will simply destroy the log rather then light it on fire. It's worth a shot though.
I'd have to use Coal. From what I hear, logs don't work that way. Also - I'm told you can just build a roadway out of coal, to use as a "wick". My extract lights upon release from the confines of a barrel/flask, if I use this method I may as well use Booze, from what I hear of booze bombs.
All this still requires magma, however, and for framerate purposes, RP purposes, simplification purposes, "trade goods being useful" purposes... it's sometimes better to
not use magma. Crazy, I know.
Also - when we can load custom things into a catapult... don't you want explosive projectiles without making the catapult a "!!catapult!!" ?
originally my extract was a venom. But until "bites" actually inject a "venom impurity" or somesuch into a layer in a creature's body, liquid-fire venom is kinda useless.