Start by making your room(s) that you want to have a fight with the combine in. A small maze type thing would be good, with low walls but high enough that you cant see over them.
Place your info_player_start, item_suit, and whatever weapons you want, and place a few npc_combine_s in a group somewhere. Select all the combines, press alt+enter, and set their squad name to something like comb_squad_1, set their weapons to something nasty, and set their number of grenades to unlimited.
Now, from the entity list, select info_node. When you run the map in the game, a web is formed of what nodes are visible from what nodes, and the AI move along this web. Ooooh, spider-like! Place the info_node's wherever there is a corner, an intersection, wherever there is some cover, anywhere the AI might find necessary. Think, "if i was an npc_combine_s, where would I want to go, to get an advantage over Gordon on this map he made?" and place nodes wherever you think a combine_s would go. Remember that they can ONLY go where you put the nodes.
Light your map and compile it.
I've found with the map i made (example map) that the combine tend to nade each other. Stupid AI, but with more room and perhaps lower walls they can throw further. I also went pretty damn overkill with the nodes, but that never hurt anyone.
Good Luck!
Related
Example Map
Serever
