While it pains me to break a fine tradition of sniping, and as a relevant link has already been posted, i'd advise Dr Freeman here to do the following:
Once you've loaded the map, bring down the console by pressing whatever's next to the "1" button, and type the following (note that the letter in the quotes is my personal preference, replace with whatever you wish.) Note that it's probably a good idea to type in "ai_disable" into the console sans quotation marks beforehand.
bind "o" "npc_create npc_combine_s"
OR
bind "p" "npc_create npc_citizen"
this will create an unarmed combine soldier/ resistance soldier at the crosshair location.
To arm the ones to follow, type the following before spawning them:
npc_create_equipment weapon_smg1
If necessary, you can replace _smg1 with _AR2, _shotgun, etc.
Pressing "o" will now spawn combine soldiers with sub-machineguns. Pressing "p" will create their counterparts in the Resistance.
Entering "ai_disable" again will cause them to fight each other, provided they are close enough.
Addendum: It's probably a good idea to use the "notarget" cheat before enabling the AI, unless you want the combine to concentrate on you to some extent. See the Super Happy spawning thread for more entities to spawn.