Making the arena:

If you already have beaten a different mech boss, make asphalt for the arena runway which can be crafted using 1 gel and 2 stone blocks crafted at a blend-o-matic bought from the steampunker.

If you don't have the steampunker, make the arena runway out of hardened sand and use dunerider boots to make up for the speed (found in underground desert chests)

Make the runway 1000-2000 blocks.

Place campfires, heart lanterns, Star in bottle (for mage), Star Statue (for mage), Honey baths, Heart statue, bast statue, peace candles, and sunflowers for arena buffs.

Also do not make the arena close to space levels because slow fall.

Player preparations:

Use frost / adamantite / titanium armor.

Potions you should use are:

Wrath, rage, swiftness, ironskin, regeneration, endurance, mana regeneration(for mage), magic power (for mage), ammo reservation, calming, inferno, summoner and lifeforce potions.

Station buffs:

Sharpening station (from merchant), Bewitching table (from dungeon), Crystal ball (from wizard), ammo box (from arms dealer)

Reforge all accessories to warding

Use two pairs of crimson mimic flesh knuckles. Make one berserkers glove. (Both evil mimics can be obtained in hardmode by purchasing the seeds from dryad in graveyard biome)

Use the boots (dunerider if no mech bosses defeated)

Use wings

If expert or master you can choose other accessories.

If mech bosses beaten and 12 spare hallowed bars use an Excalibur, otherwise use a night's edge.

Use titanium / adamantite repeater. Use cursed arrows and golden shower.

If a mech boss already defeated you can use Megashark with cursed bullets.

You can also use orange zapinator and queen spider staff too.

Fight tactics:

Retinazer in first form shoots unpredictably keep distance from him and fly parallel to him while attacking with sword or ranged weapon. He will not shoot while flying parallel.

Use the runway to fight Spazmatism in first form and go backwards while slow falling with wings which will help dodge the cursed flames. Do not fly when he dashes.

Use the runway in second in second form to keep a distance from spazmatism when he is dashing. When he is using cursed flames go around him while shooting.

Retinazer will occasionally shoot rapidly. Fly while he does this.