At a tournament that I attended this past Saturday, I found myself witnessing a problem with two colossals facing off against each other. The map that they were playing on allowed both inside and outside elevations (sorry that I didn't get the name of the map or the specific pieces, my memory is a bit foggy), and both colossals in question were "outside" to the best of my knowledge.

The attacking colossal uses pulse wave on the opposing colossal, who is on the edge of the elevated terrain. Now according to the line of fire chart, since the elevation was considered on the outside, and both figures are considered giants, that should be clear line of fire, is that right? Furthermore, since the defending colossal was on the edge of the elevation, he can be a legal target of pulse wave, is that right?

Correct in both accounts unless I'm misunderstanding something.
When Pulse Wave says it can't ignore elevated terrain it means you follow the normal LOF rules for elevated terrain, so as long as the target is on the ledge you are able to target it, collosal or otherwise.

Also note that you can draw Lines of Fire to/from colossals as if they occupied any elevation higher than the one they actually do occupy. So even if the targeted colossal had not been on the rim the attacking colossal could just have used a higher elevation to draw the LoF.

Looking at the lof chart, the only thing that blocks lof between 2 characters using Giant Size is indoor blocking or another character using Giant Size.

And mirandir has it right. A grounded Giant Size character can draw lof to elevated characters even if they're not on the rim.

That's what everybody at the venue but the person who had the defending colossal thought. Thanks for confirming my thought.