I haven't done this before, but I'll take a swing at it.
Put in a npc_combinedropship. Name it Dropship.
Put in a npc_strider. Name it Strider.
Put the strider, underneath the dropship. Now put a phys_ballsocket between the two ( preferably touching the bottom of the dropship ). Now press Alt+Enter, and set the two entities to, Dropship, and Strider.
When you want the strider to be dropped onto the ground, just trigger the ballsocket to Break.
Maybe put in a trigger for the dropship to fly through, and, OnStartTouch, ballsocket, Break.
This should work, in theory. Just note, that it's all speculative, and I can't garuntee. Let me know though.