Really the only difference of zombies between Story mode, and Arcade mode is that the Story zombies attack with their fists, and walk very slowly.  This is your advantage in Story mode.  If you know that zombies are coming, you don't have to panic.  Just kill them before they kill you.  Decapitating them is a quick and easy way, but demands accuracy, and speed.  You can keep firing at them until they die, but you might want to preserve your ammo.  Punching them is another option.  But doing so puts you at risk of being hurt (they can trick you while doing this, you might think they wont try to attack until you are close enough to smell their breath, but sometimes they use an uppercut type move).  Beware of Sewer Zombies on the Normal, and Hard levels of Notre Dame.  These zombies spit a green gooey stuff that can hurt you.

 

Well...what can I say?  The Reaper Splitters are your arch enemies, and will do anything to get rid of you.  They usually appear on Story mode levels after completing a certain objective later in the level.  They can also appear in the final level, the Space Station (Well duh, that's their spaceship).  I must admit, I always get a little on the jumpy side whenever I know they are here.  They can be hard to find, because they use a special cloaking device, and warp around the place.  They then pop out in front of you.  They don't use guns, instead they use lightning shocks.  These shocks can be avoided, but with several reaper splitters in a room, the bolts can become overwhelming.  Once you find one, make sure you get rid of it for good, or else they will follow you around like a little puppy dog.

 

What's that?  You think monkeys are so cute, and wouldnt hurt you?  Well, the cute part is up to you, but these monkey's are not friendly to you.  They only appear in story mode in the Jungle level.  As soon as you start, you see a monkey start running up the ramp.  Follow it, but keep low,  there are usually watch monkeys on high cliffs.  If they spot you, they will blow a warning horn, and the other monkeys will scatter and warn others.  Many of the monkeys you encounter are not hostile, but some you encounter will throw explosive watermelon (I've heard of the banana bomb from Worms3D but this is ridiculous)!  Make sure you look up when exploring the level, and watch out for them in the temple depths. 

 

The Drone splitters were a peaceful race of aliens, until the Timesplitters enslaved them, and sent them to kill you.  They don't use guns, they just punch you instead.  You want to avoid the baby drones too, they are a little faster.

 

The security cameras are not enemies i know, but they can get you into some jams if you are not careful.  The first mission you embark on in story mode involves invading the Russian military base by sneaking through the dam.  On the outside camp, there are security cameras that you can do three things to avoid them.  1. Sneak past them and ignore them.  This can be difficult at times, especially when you are caught in a firefight.  2. Shoot the camera lens.  This is highly recommended.  This will disable the camera for good.  3.  Turn the camera off, or even use the gun turret (if available) to kill enemies.  Usually not too far from the camera, you will find a console that you can control all the security cameras.  You can then switch the camera off, or shoot an unexpected victim with the gun.  On the Robot Factory level, the cameras actually move on rails, and can be shot down.  You must use one of these to complete the objective of overloading the electrical thingamajigs.  Remember to use the Temporal Uplink if you don't want to get caught!

 

Yeah, i know that watermelons don't pose a threat to you (except in the Jungle).  I just like them!  You can shoot them, sometimes they splatter, but sometimes they roll.  I got mine to roll down the stairs in the Siberia level!