The 'How' doesn't matter. She is a channeler. There are probably literally hundreds of ways she could have killed him that wouldn't leave a body. Everything from balefire to throwing it out through a skimming gateway to burning it to ash and sprinkling the ash into everyone's wine so you can laugh about it later. It is completely insignificant.

The 'where' is because she was probably one of those two servants Rand runs into in the hallway. There isn't a lot of evidence to back this up, but based on what we know about the royal palace and where it is described that Rand was when he ran into those two servants, it is likely that location is very close to the pantry Graendal killed Asmo in.

I really don't see why it matters that much though. This murder was not significant. Asmo had completed his purpose, and now he was just an annoying character. So he got offed. The method works into the story, but is not important in and of itself. Lets all move on please.