I thought this would be equally as interesting as the normal TFNFG thread.
It works the same way as that thread, only instead of listing weapons,
characters, etc. for characters you play as, you’re listing tiers for the
enemies you fought against.
Note : Goes without saying, but let’s be respectful. Not everyone breezes
through or has difficulty with the same enemy.
I’ll start with Quake and it’s expansions (Excluding the X-Men one,
since it was terrible.)
God
Shub-Niggurath
The funny thing is, I had the hardest time figuring out how to kill
this bitch. She doesn’t do anything. Doesn’t move, attack, nothing.
And after killing all the enemies on the stage, you’re essentially
stuck with a boss that can’t be killed via your weapons, yet doesn’t
do anything to you. There’s literally no explanation or hint given as
to how to kill this thing. I FINALLY managed to kill this thing 20
minutes after I had cleared the stage via walking into the teleporter
just as her eye went in. Honestly, who would ever think to do this? 
Top
Shambler
Had that ridiculously hard to dodge lightning bolt, would tear you
to shreads in seconds up close, and would ALWAYS seem to spawn
on you in the worst places. (-_-)
Vore
Although not as strong as the Shambler, they have one thing that
annoys the shit outta me. Their goddamn homing bomb. Sometimes
it will go around a corner and hit you! Which of course, deals a lot
of damage, and since it’s a bomb, it can do some slight splash
damage to you. One thing to note is that you can make this bomb
follow you and hit other monsters, giving you some breathing room
and even taking the heat off you.
Fiend
Moves fast, can jump really really really far, and said jump is also
a strong leaping attack that after they land from it, they just go
to town at ripping at you. Probably the only non-boss enemy to
be as equally dangerous and threatening as a boss enemy since
you have to keep moving and be very choosy as to which weapon
to use (explosives are a bad idea unless you’re in an open area or
you can trap him into a pit he can’t get out of).
Mid
Chthon
Can do a lot of damage to you if you’re hit by him, but what
puts him in mid-tier is the very easy pattern to damage him,
and that he’s actually rather simple to beat. As long as you
use some caution, you’ll have no problem. For a boss he’s
really not that bad.
Spawn
IMO, the second hardest non-boss enemy of the game. These
things move amazingly fast, can hit you hard, and eventually
they explode (or if you kill them) and do a lot of nasty splash
damage! If you have more than one chasing you, prepare to
get messed up bad.
Ogre
Very common, have grenade launchers, and a long reach with
their chainsaw attack. Not too difficult to kill since you can
knock them down, allowing them to take free damage, but they
can be quite the headache if not dealt with immediately.
Death Knight
These guys are tough, taking a good beating, dealing lots
of damage up close, having a spreading projectile, and can
move fairly well, but can still be killed without too much
difficulty compared to the higher tier’d enemies.
Bottom
Scrag
About the same as a Knight in terms of damage and durability, but
can fly and is usually in pairs or small groups, making them very
annoying. Luckily like Knights, they die without much effort.
Knight
Move somewhat fast and can do some damage, but they’re about as
durable as the Enforcer. Probably the strongest of the weakest enemies.
As long as you move backward and shoot, they actually get stumbled,
allowing you to kill them rather quickly and easily.
Zombie
Can’t be killed by non-explosive weapons unless you use a quad damage
powered double shotgun. But are ultra slow, and can be very easily
avoided. They can also be used as a great distraction. Because they
can’t be killed via explosives, a lot of enemies will just keep hacking
away at them, and not stop since the zombie can’t die.
Enforcer
A grunt that does slightly more damage, and takes slightly less damage,
but is ultimately as easy to kill.
Grunt
Can be annoying in groups, but is otherwise very very weak and
easy to kill. Can be two-shotted by the shotgun, and one shotted
by the double barrel shotgun. Any other weapon is overkill on them.
Rotfish
A fish version of a Rottweiler. Just as weak and easily killed as one
too.
Rottweiler
Little damage, and very weak. Not a threat in the slightest bit.