Hi!
I use my stationary bike for HIIT, but I have a friend who uses the elliptical. She keeps it on Level 6 the whole time. I switch between L2 and L6. For me, it's too difficult to get a quick enough recovery if I keep it L6.........for her, not. I suspect it has to do with the fact that you're standing on an ellipt.; seated on the bike. But we figured this out by trial and error.......so experiment.
The important thing with HIIT is to get your heart rate pretty dang high for 15 seconds.......move like your life depended onit, and then allow yourself to recover, but not toooo much. Then do it again.
Be sure to get a good 5 min warm up in (low level), and at least a 5min cool down (low level). So far, I can only do about 10min of actual intervals before pooping out, but I have found the L6 starting to feel a tiny bit easier.
No matter how many actual intervals you are able to do, stay on the thing for 30min total, to get the most benefit----fill in the extra time at a low level, and mybe do one more interval just before you cool down.
Have fun! True HIIT really kicks your butt, but in a good way,
Oh, and one more tip........ place your elipt. on a mat or something, cuz they have a tendency to hump along the floor when you're pushing that high level.