Aye. I've already done a number of the highlights, including Elite Fireworks (man, that stuff is funny), the Rockslide Megabattle, the Spiral Path Megabattle, etc. However, I never really got around to the harder stuff, like the Twin Bridges Megabattle or Banshee Battling, mainly because I always thought "Dear god, it's full of work! I don't want to go through all that effort just yet."

Dunno why, but I think I might as well throw down a personal list of how easy or hard each of the Megabattles are to create:

Rockslide Megabattle - Easy to create some kind of save, but it seems like it's hard to create a good save. The enemy behavior is quirky, and if you do the wrong thing, you may find the enemies still fighting the Marines or something like that. All in all, it depends a lot on enemy behavior, which is unpredictable and hard to control (or in this case, impossible, considering that you have to sit outside the area to create the megabattle).
Cavern Megabattle - A bit trickier to create than the Rockslide Megabattle, given that you have to do a Bridge Descent and then set everything up, which can take some time (forty minutes if you hurry, upwards of an hour if you're more tedious and careful about setting everything up). However, it's much more controllable than any of the other battles, relying only on one bit of enemy behavior (the retreat caused by going low over the cavern bridge), and that bit is actually very simple; pass over the bridge, they all retreat to the far side of the cavern. All in all, very easy to control and set up.
Spiral Path Megabattle - Takes a long time to set up, and relies on a bit of tricky enemy behavior. Still fairly easy.
Twin Bridges Megabattle - Takes a very long time to set up, and involves a lot of enemy manipulation. Of course, this is mostly just my assumption; if I could see a good video of it being set up (err, *hint hint*, *wink wink*), maybe I might have a better sense of it all.
Dustbowl Megabattle - Involves a lot of enemy manipulation, and requires you to dive into the line of fire like no other megabattle does. If you're playing on anything above Easy, it's probably rather tricky to set up.

Vale,
Perseus