I used the wrong terminology I suppose.

You only need to pass the last checkpoint in a chapter in insane/hardcore to get the entire chapter "passed" as insane/hardcore, everything else can be done in casual. Do this for all the chapters and you'll get the insane/hardcore achievement. It's actually a pretty well-known exploit/bug/cheat (whatever), I'm surprised not more people know about it.

[edit] ok wait a minute, I DID use the right terms.

e.g. Act 5 has 3 chapters, right? I believe the second chapter starts from berserker until just before Raam. You can play this entire part on casual, then just before entering the ammo car, quit and reload in insane and trigger the checkpoint and you'll get the achievement for insane for the second chapter.