Skeletron Prime is one of Terraria's three mechanical bosses. It has four weaponized limbs—a vice, laser, saw and cannon—but only the head must be defeated to win the encounter.
Summoning
Use a Mechanical Skull at night.
Decide whether to remove limbs
You do not always need to destroy every limb first, but removing the attack that causes the most trouble can make the rest of the fight much easier to control.
Arena preparation
- Use enough horizontal space to circle the boss.
- Add vertical movement so you can avoid overlapping limb attacks.
- Bring healing for a sustained encounter.
- Use a weapon that stays effective while moving.
Head-spin pressure
When the head becomes more aggressive, prioritize survival and spacing over squeezing in extra damage. Losing one attack window is better than getting trapped inside several overlapping hitboxes.
Mechanical-boss progression
Choose between Skeletron Prime, The Twins and The Destroyer based on what your current build handles best rather than following a fixed universal order.
Source: Official Terraria Wiki.