Put Burning Flames That Deal Dmg/sec On Ground?

You know how flame strike does that gong sound and the twirly graphics.. then does the burning a delayed bit later? I want to do just the flames on the ground instantly but i am not sure what the best way is to go about this.

actually, burning oil would be more accurate to what i'm looking for. i like the way those flames look on the ground as an afteraffect

my only guess is putting a dummy unit with an aura that deducts health like immolation and using a special effect function to display the flames

is that my best bet?
how does icefrog do it with Lucifer's flames on the ground

That's actually the simplest way.
Make a unit whose model is the burning oil effect, and give it a passive immolation aura. You pretty much answered your own question.

Well the reason Flamestrike is delayed is it has a Cast time of one second by default. if you reduce it to 0, you can get the effect instantly.