Gather as much as you can in healing potions and ironskin potions. Make sure your defense is high, for both eyes do high damage to you in their second forms. Further, gather and set up some heart statue connections to wires so that you can regain some of your health. Make sure you use pressure plates and not a timer, as the timer will only go on a scheduled timeframe.

First, build an arena about 150 blocks wide. Ensure that you have a 1-2 block high, 10 block wide bump on either side of the arena. This is to prevent Spazmatism's Cursed Flames from getting to you, as they do not travel through solid blocks(you will still be able to fire on him when he is trying to spew his fires). Don't worry about Retinazer until his second form; in his first, he'll only hit you with his lasers a quarter of the time he fires them. 

Weapons

Make sure you have an Unreal Megashark with Cursed Bullets, so kill The Destroyer first. He's pretty easy, so you shouldn't have a problem if you have good defense(around 50 or so) and some good weapons and some tactics to evade him. Another tactic is to use the Star Cannon, but this does not inflict the Cursed Inferno debuff on Retinazer or Spazmatism. Equip a Ranger Emblem, preferrably a Menacing one, and make sure all your accessories are Menacing or Angry. 

Armor

Adamantite armor with the ranged head piece is the best, but going for the melee head piece provides more defense. 

The Battle

Start the battle immediately when the sun sets. They will come at you and begin swinging rapidly. Spazmatism will usually go to the left side of the arena and attempt to fire Cursed Flames at you, but will fail because of the bumps you placed. Fire your bullets from the Unreal Megashark and attempt to evade the passes made at you from Spazmatism. Also try to evade the lasers shot from Retinazer. If all goes well, you should still have around 375 health by the time Spazmatism goes to his second form. The same thing will occur in his second form; he will attempt to spew his fires at you to no avail. Obviously, when Spazmatism dies, your target will become Retinazer. He won't be a problem until he transforms, and when he does, evade his rapid-fire lasers as well as you can. Once this is accomplished, you've done it!