Finish the College of Winterhold questline until the final fight in the Hall of Elements
Equip strong magic resistance and shock resistance gear
Bring healing potions and magicka potions
Use wards or spell absorption if available
Destroy the Eye of Magnus with the Staff of Magnus
Keep the Staff of Magnus on Ancano and the Eye at all times
Attack Ancano only after the Eye is weakened
Reapply the Staff of Magnus beam whenever the Eye starts glowing strongly
Avoid standing in the center of the room when the Eye is active
Use ranged attacks, summons, or shouts while maintaining pressure
Interrupt Ancano if he begins casting dangerous spells
Stay mobile and circle the room to avoid damage
Save before entering the Hall of Elements
Lower difficulty if needed