Complete the College of Winterhold quest line until the Eye of Magnus fight
Bring the Staff of Magnus
Equip strong magic resistance gear
Equip shock resistance gear
Bring healing potions
Bring magicka potions
Use the Staff of Magnus on the Eye of Magnus
Keep Ancano away from the Eye while it is active
Attack Ancano only after the Eye is closed
Repeat the cycle until Ancano is defeated
Use followers or summons to distract him
Use ranged attacks if you are struggling in melee
Save before entering the fight
Lower the difficulty if needed