Secret Mission 6 is actually terrible.

[LaSaia]

Why would anyone think something like this was a good idea to put in the game?

[Simon]

Lmao, I just googled for it.

[ShinkuTear]

They thought Gattuso was okay, especially with his SM. They thought making aprox 30%-40% of Part 1 rainy and gray and depressing was okay. An SM where the only difficulty is having the correct character is of course going to be okay when it's nowhere near as bad.

Needing a specific character for an SM without clear in-game notice can be pesky and I will agree with you on that, but I would argue is also much less terrible than the mere existence of Gattuso or some much later SMs.

At least you only need to use the correct arte in the vaguely correct location, shortly before the boss goes and heals. The arte should last a few seconds, giving you some freedom for this.

[LaSaia]

I didn't really have any trouble with Gattuso or his secret mission and got it fairly easily on my first attempt on Hard. Pretty easy to just wait by a flower and guard until he does a rush attack at you, then hit the flower. Even if it's not easy for some people, there's still not a whole lot of luck involved.

With Gigalarva, which I did finally manage to do the secret mssion on, you have to keep the boss below a certain health threshold without killing it and then have a ton of variables go in your favor to get the secret mission done.

The hitbox of the trap is tiny, so it is impossible to use it offensively. Your only option is to lay the trap and pray that the boss goes to the right spot to try and heal itself.

But wait, it's not just as simple as that, either. The traps disappear over time, and their startup animation is slower than almost the entire duration of the boss's healing animation, so you have to be in the spot spamming the trap in order to have a chance of it working.

"Okay so get it below half health and just spam traps at the edge of the pool." Except again, it isn't that easy. The boss can still decide to do its spin attack or go into overlimit before trying to heal, which will send you flying. And due to the slow startup of the trap, it can do these before you have time to block or move to prevent yourself being knocked away. If you do get knocked away, you trap is probably going to disappear and the boss is going to heal itself uninterrupted.

Now then, the boss itself, Gigalarva, is not hard. I wasn't really at any risk of death at any point during this. The reason I kept failing to do the secret mission and having to reload was because of eventually running out of items to restore TP and HP with. No TP = no traps and no healing from Estelle. And so you do eventually, very slowly, lose the fight. If the secret mission for this boss where skill based like literally any of the other secret missions have been up to this point, perhaps at the expense of making the Gigalarva fight actually challenging and dangerous, I wouldn't have a problem with it. Instead, though, it's pure luck with a number of variables that the game has to decide will go in your favor in a fight that is not even remotely difficult.