I have not done this yet, but Demons would not let me sleep until I got up to type this out. :\
Be creative: you can do many things.
Know exactly where his lungs are located, from a picture/diagram on the net (medical pic.)
You can envision two fireballs, forming them with breath and from your palms and when they are hot and crackling enough, stuff them into his lungs, either in person when you are in the same room. Or from a picture of him.
You can also do this just by envisioning him, but doing it in person or from a pic has less distractions (if he has strong protection or if you get attacks.)
Another method is to astrally insert two tubes, one into each of his lungs, and breathe fire into them. Just make sure you don't inhale back into yourself.
You can also do damage by astrally cutting up his lungs with a knife, or poking holes into it with a hot poker (like something from the fireplace.)
Stuff straws into his lungs, make him like a big organ, where you play the keys. ^.^
Pump fire in, watch fire consume his internal organs and body.
Or fill it with <icy water too, watch him choke to death.
Since We are on the topic of this, if this does not work, you can always squeeze his heart astrally. (Or pump it like a ball to relieve stress, that's your call.)
An acquaintance of mine did this once to his ex, and she actually had heart palpitations and went directly to the ER.
You can always program his aura, to continuously invoke fire into his internal body. Or attach tubes into his lungs that are attached to an astral fire.
The last method, is a variation on a method the Demons told me about some time ago, in regards to a question a friend had:
Q) How do I expose this guy's true nature and lies to the world? I want him to slowly suffer, and die, but before he does, I want the world to see what kind of person he is.
A) -Tear out his throat chakra. Or tear out some of the fan blades, a little every so often. (Depends on how slowly you want him to suffer.)
-Program his aura to show the true nature of his being to all he comes into contact with.
-Pack said aura with your choice(s) of colour.
-Tear holes into his aura.
-Attach enormous tubes into his heart and throat chakra, that are attached to raw sewage and all matters of indescribable filth. You can do this, attaching these tubes to an actual place on the physical, or make your own place on the astral. Or, if you find a realm on the astral, that contains all this, link him directly.
For someone who spews shit, fill him with it.
Hail Satan!
Hail the True Gods of Hell!
Hail all the Demons and the Bes!