Skeletron Prime is the Hard mode version of Skeletron. He can be summoned with the Mechanical Skull anywhere at night. Skeletron Prime has four hands, each of which functions differently, and his head. The battle is over when you deal enough damage to the head.
NOTE: The Mechanical Bosses can only be summoned after dusk (7:30PM) and they must be destroyed before dawn (4:30 AM). It is also wise to spend the time making an Arena as a place to fight the Hard Mode Bosses, especially if you going after them solo. You will also want to utilize potions and buffs, such as Ironskin, regeneration, Thorns, etc. to make the battle easier on you.
RECIPE - craft at Mythril/Orichalum Anvil
5 Souls of Night
5 Souls of Light
5 Copper Bars
5 Iron Bars
1. Use a weapon that penetrates, such as a flail, spear, Death Sickle, or magic spells in order to hit multiple arms with each attack. this will help damage the head part while going for the hands.
2. THE HANDS - The Prime Laser shoots lasers and specifically aims for you (This should be your first priority). The Prime Saw swipes back and forth dealing large melee damage (second priority). The Prime Cannon fires grenades randomly (third priority). The Prime Vice swipes around randomly (lowest priority).
3. Wear sturdy armor (Turtle preferred) since it is large and you will get hit a lot. Also might be wise to wear a Cobalt/Ahnk Shield
***Same as 360 version***