Triangle planes in level editor?

[The chaser]

You mean 45?

Anyway, the Level Editor is not really designed for modeling - it's a tool for setting up game levels (including engine-specific helper objects like triggers, sound entities, etc...). If you want to create more complicated models, you should use a 3rd party modeling app, like Maya, or 3ds Max, or Blender (which is free), export to COLLADA (dae) format, and use those.

But if you want to create custom monsters and such, you can do it only with Maya as far as I'm aware.

Er... modelling isn't the question. I don't think he wants to handle modelling programs, so just 45 degrees on the plane should work.