What's the one boss harder than Culex? Culex! ...Wait--
The Nintendo Switch remake of Super Mario RPG added a handful of harder rematches against various bosses that can be fought after defeating Smithy for the first time. The very last of these rematches is Culex! ...Or rather, Culex 3D, who has achieved his dream of becoming three-dimensional thanks to the power of Mario and friends' power and the fulfilled wishes from across the land. And let's not forget those pesky crystals, which have also attained the third dimension! Just like last time, many of the attacks you'll be faced with are unblockable, but fear not! Defeating him is possible! Here's what I did to defeat him!
Prerequisites
In order to be able to access the post-game bosses at all, you must first stay the night in Marrymore's suite with the Stay Voucher Toad gives you immediately after beating Smithy for the first time and loading your save file. Note!!! If you didn't save your data after defeating Smithy and watching the credits, you won't have access to the post-game content! The game tells you this, so it's on you if you don't save.
As far as I know, the post-game rematches have to be done in the order the Frog Sage gives you hints for each in. This order is Scrathy-Throat Belome (in Belome Temple), Levelled-Up Punchinello (same place as first fought in Mole Mines), Engine 023 Booster (in his room at the top of Booster Tower), Extra-Fancy Bundt and the Exhausted Tortes (same place as first fought in Marrymore), and Duel-Ready Johnny (same place as first fought in the Sunken Ship). Mario-Style Jinx is also part of these rematches, but I found you can actually rematch him at any time. At the very least, you have to defeat Duel-Ready Johnny in order to get the Extra-Shiny Stone from him. Without it, the door to Culex's room won't reappear if you've already beaten him.
Levels, Items and Equipment
When I went to go fight Culex 3D, my party was at level 27, and I had done all the previous boss rematches. I had done a small amount of grinding for a couple of levels in Weapon World for the Booster rematch, but I feel like it's entirely possible to do all the boss fight rematches, including Culex 3D, at the same level you defeated Smithy at. For me that level was 25.
For recovery, you'll want to stock up on the same items you did for the first Culex fight, albeit with some alterations. Since there's no choice but to do this fight after Smithy's, you'll be able to buy Max Mushrooms from the Toad stationed in the Factory. Since you'll be able to get as many Max Mushrooms as you'd like, Mid Mushrooms are less of a concern, but I'd still advise stocking up on however many you feel you might need. Get your hands on some Maple Syrups too! Croaka Colas are quite helpful for the early rounds of this fight, so stock up on those! While not sold in any stores, there are a few Royal Syrups throughout Bowser's Keep, and one in the locked room in Belome Temple. Getting the key for that room is a bit of a process - you need to do at least 30 consecutive Super Jumps on any one enemy, talk to the Chow in the house next to Monstermama's in Monstrotown, exit, then go to Belome Temple and hit the Belome head before the elevator. If it reads "Sorry, I'm not accepting visitors past my bedtime," you've got the one that'll lead you to the locked room.
Equipment wasn't as hard for me to figure out, as I also didn't have as much trouble with the fight overall! Like last time, everyone should have their strongest weapons and armour equipped, but here's what I did for accessories.
Mario got the Attack Scarf, which can be obtained from the Chow in Monstro Town as a reward for doing 30 consecutive Super Jumps. I'd actually put this on for the Johnny rematch, but it came in handy here too! The higher stats meant a lot in the long run. If you don't have the Attack Scarf yet, I'd recommend jumping on Goombas in Mushroom Way. And also having an amount of FP that's a multiple of 7.
Mallow got the Quartz Charm! This, apparently, halves damage taken, adds 50% to attacks performed and protects against mortal blows. Maybe this would have been a bit better on someone else, but Mallow did come in handy on occasion!
Geno got the Jinx Belt! Apparently it too protects against mortal blows! Plus, the extra attack is helpful once Culex is on his own.
Bowser kept Booster's Charm. Apparently you can get it from the Booster rematch if you failed to get it from the curtain minigame.
And Peach kept the Safety Badge! Again, you'll want to keep her safe from statuses so she can heal up the rest of your active party.
The Fight
Like last time, I opened the fight with Mario, Bowser and Geno to do as much damage to as many enemies as possible. A couple of things to note for this fight: Culex 3D starts the fight off with the crystals, so he doesn't have to spend a turn summoning them, and you can't outspeed him to get a potshot on him. Also, he is now faster, and will open the fight by using Meteor, an unblockable move that leaves the whole party at 1 HP, at least on this first usage. I've had it do a bit more or a bit less on later uses. You'll want to use a Croaka Cola as soon as possible to get everyone back up to full, then use Geno and Bowser's turns to do some multi-target damage.
From here, Culex will use Meteor every five turns - there will be an indicator that goes from 4 to 0 on his turns. Heal up on the turn before 0 so you don't get destroyed on his next turn! Other than that, the strategy is mostly the same from the first Culex fight: use multi-target attacks, heal when necessary. But don't use any Triple Moves when the crystals are present!
The Wind Crystal will use Petal Blast pretty soon after your first turn, so you'll want to swap into Peach to get everyone else back to normal. Keep her out - even if you're good enough at timing blocks to not need to heal as often, Psych Bomb is still a pretty useful move to be using when you can. Like with the last strategy writeup, I recommend using Ultra Fireball on Mario's attacking turns. If Geno falls when the crystals are still out, Mallow is a good switch-in! Using Snowy can get some good damage in on the Fire Crystal to compensate for its resistance to Fire.
Occasionally, one of the crystals will lock a specific button's commands for a few turns, a la the Bowyer fight. These can range from annoying to scary depending on context, but arguable the scariest is Item Lock. Not only can it stop you from recovering FP, it can also stop you from reviving party members if Peach is down. If you Special Lock, I'd recommend going after the Wind Crystal. I believe that crystal's the only one who can do it.
When a crystal is defeated, it will give all other enemies a buff before shattering. These buffs are Vigor Force for the Fire Crystal (buffs Attack), Magic Force for the Water Crystal (buffs Magic, not sure if it applies to Magic Attack, Defense or both), Valor Force for the Earth Crystal (buffs Defense) and Nimble Force for the Wind Crystal (buffs Speed).
Once all the crystals have been shattered, Culex starts using Final Claw instead of Meteor when his counter reaches 0. This attack does 9999 damage if you fail to block it, and (I believe, don't quote me on this) leaves you at 1 HP if blocked. This is why you'll want to save your Triple Moves for Culex on his own - Spare-Us-All is fantastic for countering Final Claw, and it'll guard two other party members from whatever attack he uses afterwards! Keep in mind you won't be able to use Spare-Us-All for every Final Claw, but it should be a good protection against at least a few.
As for attack Culex on his own, you'll want to switch to Ultra Jump on Mario. If you got the Attack Scarf like I did, you should be well practiced in timing the jumps. If you don't want to use Psych Bomb on turns where Peach doesn't need to heal anyone, you could switch into Mallow or Bowser, provided they're still alive, and attack with them. I'll tell you this, though - Bowser's damage never left the 100-120's range for me, so you might want to use Snowy or Star Rain with Mallow and see how that goes. Just make sure to switch Peach back in for Spare-Us-All before Final Claw! Geno's Attack stat is massive, so simply attacking Culex with a regular attack should be good. Well timed Geno attacks did around 370 damage for me.
And like last time, avoid using Thought Peek. Monster List nuts like me will need another 15+ Frog Coins.
After the Fight
Once you defeat Culex 3D, he will revert back to his 2D form and have a short conversation with you. He'll even tell you how many turns it took to defeat him! He'll give you a Crystal Shard for defeating him - this item's just a novelty to say, "Hey! You beat the strongest boss in the game!" Unlike the first fight, however, the door to his room will not disappear, meaning you can fight him as many times you want!
Congratulations! You've just defeated Culex 3D, and done everything in the Super Mario RPG remake! Monster List nuts like me should head to Booster Pass to get the rematch's Thought Peeks and get 100% completion on their Monster Lists.
Have fun, and hope this helped!