Have you tried using an npc_maker entity or an npc_template_maker entity (more complicated but let you have them spawn and do stuff - if you can get it work)

NPC_MAKER

You need to give the entity a name for this!

Make one where you want them to spawn. Orientate it to face the direction you want your spawning monsters to face.

In the [Class name of spawned NPC] box type the name of your monster (ie npc_headcrab) etc

[Frequency] Should be obvious - the time it takes for a new monster to spawn after the previous one died (I think a setting of -1 makes the next one spawn when the previous one dies - I read it somewhere)

[Max Live NPCs] Maximum number of NPCs alive at one time (-1 = infinite)

[Num. of NPCs] The maxiumum number of monsters that can ever be spawned (-1 = infinite)

[Children's name] The name that will be given to every spawned monster

[Children's squad name] The name for the spawning monsters' squad

[Weapons] Obvious. The weapons given to the spawned monsters. YOU CANNOT PICK UP THE STUN-STICK!

[Start Disabled] Only have as yes if you want them to spawn as soon as the game begins. If you wan it to stop you will have to mess with the [Frequency] and [Num. of NPCs] boxes

NPC_TEMPLATE_MAKER

Like the NPC_MAKER but instead you make your desired monster, fiddle with its settings and check the Template NPC flag and give it a name

Then when you make your NPC_Template_Maker you enter your npc in its [Name of Template_NPC] box

If you want this thing to trigger when you walk into a room you have a trigger_once brush entity in the doorway. You give it two outputs - one to turn it on, and one to turn it off after a delay.

Anyone want this as a tutorial?