Make sure you have cheats on.
To do this open the console and press "~" then type in sv_cheats 1 and press enter.
Then to spawn in the enemy you want open up the console and put in any of the npc's tags in the guide below (the thing that starts with npc_create) and press enter.

For quick spawning use the bind command so you don't have to keep typing in spawn tags.

impulse 101
- gives player all weapons in ammo

notarget
- everone ignores you

noclip
- disables collision and allows flight

god
- enables godmode

bind "(letter)" "(command)"
- Whenever you press the letter you put in the command the command you put in will activate (EX. bind "x" "npc_create_equipment weapon_glock;npc_create npc_human_security" will make it so whenever you press x a security guard with a glock spawns)

Scientist










npc_create npc_human_scientist
- male scientist

npc_create npc_human_scientist_female
- female scientist

Security










npc_create_equipment weapon_glock;npc_create npc_human_security
- security guard with glock

npc_create_equipment weapon_357;npc_create npc_human_security
- security guard with magnum

npc_create_equipment weapon_mp5;npc_create npc_human_security
- security guard with mp5

npc_create_equipment weapon_shotgun;npc_create npc_human_security
- security guard with shotgun

npc_create_equipment weapon_rpg;npc_create npc_human_security
- security guard with rpg


gman










npc_create npc_gman
- invincible

HECU grunt










npc_create_equipment weapon_mp5;npc_create npc_human_grunt
- Gas mask HECU marine with mp5
npc_create_equipment weapon_shotgun;npc_create npc_human_grunt
- Gas mask HECU marine with shotgun
npc_create_equipment weapon_rpg;npc_create npc_human_grunt
- Gas mask HECU marine with rpg

HECU medic










npc_create_equipment weapon_mp5;npc_create npc_human_medic
- HECU medic with mp5
npc_create_equipment weapon_shotgun;npc_create npc_human_medic
- HECU medic with shotgun
npc_create_equipment weapon_rpg;npc_create npc_human_medic
- HECU medic with rpg


Grenadier










npc_create_equipment weapon_mp5;npc_create npc_human_grenadier
- looks just like grunts but can launch grenades with mp5


Commander










npc_create_equipment weapon_mp5;npc_create npc_human_commander
- HECU marine with crimson beret and mp5
npc_create_equipment weapon_shotgun;npc_create npc_human_commander
- HECU marine with crimson beret and shotgun
npc_create_equipment weapon_rpg;npc_create npc_human_commander
- HECU marine with crimson beret and rpg


Sniper










npc_create npc_sniper
- Can only be killed with explosives.

LAV










npc_create npc_lav
-Armored vehicle that has a heavy machinegun

M1A1










npc_create npc_abrams
-Heavily armored tank with main cannon and lmg


Osprey










npc_create npc_osprey
- a large helicopter that can be shot down


Apache










npc_create npc_apache
- a heavily armored chopper with machineguns and rockets

Female assassin










npc_create npc_human_assassin
- an acrobatic woman useing two pistols

Headcrab










npc_create npc_headcrab
- a pesky headcrab
npc_create npc_headcrab_poison
- poison headcrab form hl2
npc_create npc_headcrab_fast
- fast headcrab form hl2

Zombie scientist










npc_create npc_zombie_scientist
- Zombie scientist

npc_create npc_zombie_scientist_torso
- Zombie scientist (crawler)


Zombie security










npc_create npc_zombie_security
- Zombie security guard


Zombie HECU marine










npc_create npc_zombie_grunt
- A zombie marine

npc_create npc_zombie_grunt_torso
- A zombie marine crawler

Zombie HEV










- Makes noises

Alien Grunt











npc_create npc_alien_grunt
- Armored buff alien that uses a hive hand
npc_create npc_alien_grunt_elite
- A bigger grunt
npc_create npc_alien_grunt_melee
- Has no armor and no hivehand

Alien slave











npc_create npc_alien_slave
- Alien slave that shoots lightning
npc_create npc_xort
- Friendly vortigaunt

Snark










npc_create npc_snark
- pesky little monters that only survive for a short time



Gargantua










npc_create npc_gargantua
- an invincible armored creature
- use command sk_gargantua_debug_allow_death 1
to make it not invincible


Ichtyosaur










npc_create npc_ichtyosaur
- a water creature

Leech










- AI is very broken

Bullsquid










npc_create npc_bullsquid
- An acid shooting squid


Houndeye










npc_create npc_houndeye
- a dog that creates shockwave


Suicide houndeye










- A red hound eye that explodes

Tentacle










npc_create npc_tentacle
- wouldn't recomend spawning this but you can if you want to

Ground sentry










npc_create npc_sentry_ground
-ground sentry that attacks everything


Ceiling sentry
npc_create npc_sentry_ceiling
-a sentry to go on a ceiling (kinda glitchy)

npc_create npc_crow










- A crow that has no reactions


npc_create npc_pigeon










- A pigeon with no reactions


npc_create npc_seagull










- A seagull with no reactions

Can be given to spawned entities
weapon_assassin_glock
weapon_crossbow
weapon_crowbar
weapon_frag
weapon_glock
weapon_gluon
weapon_headcrab
weapon_hivehand
weapon_mp5
weapon_rpg
weapon_satchel
weapon_shotgun
weapon_snark
weapon_tau
weapon_tripmine

NOTE: Some weapons with either not work or have no animation for certain entities