First set up an "npc_combine_s" Set all you variables. At the bottom there are three options you need to notice: "Waiting To Repel [Yes/No]" "Weapon [Choices]" and "Body [Choices]" (The "Elite" is the white combine. "Nova Prospeckt" is the same as the default you see in Hammer, but with 'Nova Prospeckt' on the back of his vest.) After you set everything place the soldier where you want him to START repelling from. (ie: the ceiling). Now you need to trigger a "begin repel" with another entity. This can be ANYTHING. You can have it do it when you open a door, flip a switch, kill another guard... etc. Just put it in the output section of the entity you choose and have it point to the name of the npc_combine_s you just set up. When the requirement is met, you will then see your Combine repel to the ground and take action. (Note: The combine IS visable before he repels, so you might want to put him someplace that isn't visable to the player before he repels. (My personal oppinion there.)) I have done this quite abit, so if you need me to go over something or make up a simple map, I'd be happy to. Hope this helps you out. See ya.
You need to set an NPC combine key value for ready to rappel:yes then when you want them to rappel send them a begin rappel command they will drop down until they hit the ground. The other way I suppose is to use scripted sequences.