Dalek

Daleks were the mutated descendants of the Kaleds of the planet Skaro. They fought the Time Lords in the Last Great Time War, ending in the near-total destruction of both races. Regarded by the Doctor as his greatest enemy, the Daleks were hated and feared throughout time and space. They were the oldest and most frequent foes of the Doctor, having faced him several times in all of his incarnations.

Although the Daleks looked entirely mechanical, they were in fact cybernetic organisms or cyborgs, with a biological body sheathed in and supported by a protective outer shell of Dalekanium metal armour, armed and mobile. In this respect they were somewhat similar to a Cyberman; unlike them, the Daleks' bodies had mutated so drastically from their Kaled ancestors they had lost all humanoid appearance, save for one eye (see below).

(Taken from the Doctor who wiki)

Brief History of Creation
On the planet Skaro, in the year 1450, during the final days of the Thousand Year War (REF:  Doctor Who: The Dalek Handbook ) between the Thals and Kaleds, both sides suffered mutations caused by nuclear, biological and chemical agents. Some of the mutant survivors, the mutoes, survived in the wastelands (DW: Genesis of the Daleks). Shan, a young Kaled scientist, authored a paper. With both Kaleds and Thals competing for resources, she called the only way out of this dilemma "the Dalek Solution". Davros presented the paper to the Kaled Council as his own (BFA: Davros). Another account says that Davros found a prophecy in the forbidden  Book of Predictions  which stated that one day mortals would transcend into gods. The last word, said aloud, sounded like dal ek. (BFD: Guilt)

Later, Davros, now crippled,became one of the Kaled Scientific Elite. He had begun experiments on living subjects and hoped to deify the Kaled race. Davros pushed through legislation enabling authority (and ownership) of all Kaled infants under the age of five years old and to be delivered to Pediatric Facility K99, which he used as a laboratory for surgical experiments. Davros transplanted the brain from Baran, a captured Thal spy, into a Mark I Travel Machine. (BFD: Guilt)

Davros did not immediately show the results of his Dalek experiments to the Kaled Scientific Elite. He improved and developed the shell for the organic components of the Daleks, housing them in tank-like and armed Mark III Travel Machines similar to his own life support chair. He maintained a nursery of embryonic Dalek young. As well as nurturing the physical form of his creations, Davros shaped their minds. The Daleks did not understand concepts such as pity. It did not exist in their "vocabulary banks".

(Taken from the doctor who wiki)

Battle Agianst the Big Daddies
5 dalek's are beamed down from the crucible to rapture. They have been given orders to learn about the area. Four of them are regular daleks's and thier leader a special weapons dalek. It's not long before they come across a little sister, who screams at the top of her lungs at the strange new creatures. "CEASE YOUR SCREAMING" the special weapon's dalek shouts but the little sister does not stop and tries to escape. "EXTERMINATE" one of the subordinate dalek's shouts and fires his gun stick, striking the little sister in the back and sending her sprawling.

The screams echo through the silent streets and houses of rapture, piercing the the ears of subject delta. He immediatly turns his head to locate the source of the noise. As he makes his way to where he hears the noise coming from, he is joined by 4 other big daddies also alerted to the screams. As they grow nearer they hear a loud echoing voice saying "EXTERMINATE". A nearby street glows with a blue light before all goes silent. The big daddies get to the street with subject delta at the front.

He finds the limp body of the little sister, not dead but badley hurt. Delta looks up and spots the 5 dalek's, immediatly rasing his spear gun at them. "HOSTILE LIFEFORMS DETECTED" one of the dalek's shouts, who differates from his fellow dalek's by having a claw arm. The 5 big daddies charge at the dalek's with subject delta bringing up the rear. The special weapons dalek starts to charge up his energy cannon. Subject delta notices this and leaps behind cover with too other big daddies following his lead. The other two don't notice the charges up cannon and keep on charging, almost upon the dalek's. They are just about to collide with the dalek's when the special weapons dalek fires his cannon, disintergrating both of the big daddies. (5-3)

Seeing the iminate threat is over subject delta comes out from behind the cover of a doorway and fires a spear at the dalek's but it bounces off thier shields. The other big daddies also emerge from their respective cover spots and open fire with thier rivet guns while delta loads up another spear. The dalek's also open fire on the big daddies yells of "Exterminate" filling the street's of rapture with noise. However they fail to land a hit on the big daddies who are ddodging all thier shots. Delta fires another spear, which again does nothing to the dalek's over sheild. He takes out a rocket arrow and loads it up, just in time to dodge another shot from the ebergy cannon. Aiming for the cannon itself, delta fires the rocket spear into the cannon barrel which explodes on contect with the dalek creature inside destroying the special weapon's dalek in a firey mess. (4-3)

This immediatly gives the other two big daddies a confidence boost and they fire even faster at the dalek's occasionaly pausing to load up another magazine. The dalek's are unfased by the loss of thier leader and carry on firing at the hostile native's. Subject delta back away from the fire fight and takes out a winter blast plasmid. Injecting it into his arm his hand immediatly turns to ice and he flexes his hand. He then walks back to the firefight, where the big daddies have still failed to take any damage. Delta targets the nearest dalek's, freezing it in a matter of seconds. He then loads up another rocket spear, fixes the sights and fires at the frozen dalek's, shattering it into a million pieces. (3-3)

Now thier numbers are equal the big daddies confidence soars again and they start to make thier way closer to the dalek's. However thier run of look soon ends as one of the big daddies is hit with agun stick blast and falls to the ground, stone dead. (3-2)

Delta realises that this tactic is not going to work and retreats, still firing untill he dissapears into a room. The other big daddy copies delta but goes into a different room. The two front dalek's turn to the next highest ranking dalek', the clawed one. He did not take part in the firefight as not only does he have a a claw intead of a sucker he also has a flamethrower instead of a regular gun stick. "DALEKS WITH OUR LEADER DEAD I AM NOW IN CHARGE OF THIS SQUAD" the claw dalek shouts "DALEK'S WILL PURSUE HOSTILE THREAT'S". "WE OBEY" each of the other dalek's say not once questioning the order. They split to follow the the big daddies with the claw dalek not far behind. One of them enters a room and is struck by a big daddy, which snaps his eyestalk off. "VISION IMPARED" the dalek shouts and he spins round in a circle, lowering his over sheilds. The big daddy ramms his drill into the dalek's armour and ,with no overshield, he easily drills through the dalekanium and kills the dalek creature inside. (2-2)

He turns to leave the room but is met with the claw dalek. "EXTERMINATE" it shouts and burns the big daddy to a crisp. (2-1)

Meanwhile subject delta is hiding in another room and hears a dalek approching his position. He takes out another plasmid and injects himself with it, his hand immediatly bursting into flames. He waits untill the dalek fully enters the room before jumping out and sending flames flying at the dalek. Unfortunatly the flames have no effect on the dalek other than shocking it. However it doesn't stop it from firing it's gunstick but it's aim is off and it only hits the wall behind. Delta takes advantage of this and quick as a flash grabs the gunstick and tears it out of it's socket and tossing it aside. He then grabs the dalekanium casing and tears it open, revealing the creature inside. He pulls the creature out of the casing and holds it in it's hand untill it burns from the incinerate plasmid. (1-1)

Realising how usless the incinerate plasmid is against dalek's he takes out his final plasmid, the telekinesis plasmid, and injects it into his arm. He walks out of the room and spots the last dalek, which immediatly fires at him but delta nimbly dodges and grabs a rivet gun from a big daddy corpse and fires back but as usual the rivets are absorbed by the armour. Delta soon realises that this is getting him nowhere and scans the area for anything that could give him the advantage. He spots a metal bar proped up against a wall and using his telekinesis picks it up and hurls it at the dalek, smashing into it's eyestalk and snapping it off. "VISION IMPARED" the dalek screams and delta runs over and rams his drill into the dalek's chest ripping though the overshield in less than half a minete and is soon onto the dalekanium. That dalek's tries to hit the subject delta with it's claw but it's attempt is met with failure and it's casing soon lays ripped open with bits of dalek everywhere. (0-1)

Delta dusts himself off and picks up a neraby rivet gun, just as a group of splicer's appear, attracted by the commotion. Without saying a word delta raises his gun and fires at the splicers. Winner big daddies

Expert's Opinion
While the dalek's had more advanaced weaponry and better armour the big daddies speed helped them avoid thier long range attacks and once in close range they dominated the dalek's.