Skeletron Premier is the War Mode upgrade to Skeletron and Skeletron Prime. He is summoned with a . He has six limbs, each of which has its own attack pattern. He has a laser, a cannon, a gun, a sword, a flail and a saw. Only the head must be destroyed to defeat the boss.
By summoning and defeating a boss called the Wall of Flesh for the first time, the player activates "hardmode". Hardmode causes drastic changes to the player's world. A much larger part of the world becomes corrupted by the world's evil biome and the new Hallow biome emerges. The Hallow and Corruption/Crimson biomes also spread much more quickly over the player's world until the boss Plantera's defeat, which slows down the rate of growth considerably. This change to hardmode also adds many new and much stronger enemies throughout the world, as well as new NPCs, bosses, ores, and items available for crafting or acquiring from enemies and boss drops. Three new bosses you can fight upon entering Hardmode are the Destroyer, the Twins, and Skeletron Prime, which are upgraded mechanical versions of the pre-hardmode bosses. Once the player defeats all the mechanical bosses, pink bulbs will start to spawn in the underground jungle. Destroying these bulbs will summon Plantera, a giant plant-like boss that chases and shoots deadly thorns at the player. Defeating Plantera will drop a Temple Key which can be used to grant access to the Lihzahrd Temple located in the jungle.
Skeletron Prime is a bit of a tougher customer to handle due to its arms getting in the way. However, since its movement can be predicted with relative ease, put Crystal Shots in its path with the Megashark, and it'll keep taking extra damage. If you can, try removing the Prime Laser, as there'll be fewer things for your shots to hit, and you won't have stray laser fire hitting you every so often.