The target practice is mostly for fun, but here is how to do it: Go through the level as normal or use noclip to reach the high platforms. Once you've reached the high platforms, you will need to use noclip. Fly from one platform to the next and then place a few enemies on each: npc_combine_s, npc_metropolice, npc_zombie, or npc_stalker. Just be warned that the stalker models freak out when you kill them. It's kinda disturbing, as are the stalkers.
You can even stack them on top of one another by going above the first enemy and aiming the crosshairs at the top of his head and then activating a new npc command. Once you have the enemies set up, go back to the bottom platform and pull out your favorite weapon and fire away.
Here are a couple of weapons cheats:
sk_plr_dmg_pistol 5
sk_max_crossbow 10
These are the default values, the command edits the amount of damage done, and the second command edits ammo, just replace the numbers to whatever you want. You can also see on the drop down list all the other weapons available and can scroll through them with the arrow keys.
