I haven't played FFX myself, so I'll talk instead about a fairly similar boss in Curse of Saria.
Dark Transient has an entirely fixed pattern of attacks, which I don't remember exactly, but it's something like: physical, Hellspawn (Zombie to all), Pulse (magical attack), Trauma (chance of various statuses), physical, Dark Rain (instant death to all). Similar gimmick: you have to leave Zombie on, because instant-death attacks don't work on the undead. (It's no surprise to me to learn that FFX has a similar boss; the creator of Curse of Saria has said that it's his favourite FFX, and there are many ways in which Curse of Saria takes clear inspiration from it.)
Now, the creator thought that "instant-death attacks don't work on the undead" was a common-knowledge thing players would understand, just like "healing spells harm the undead". That didn't work out; I didn't have this supposedly "common" knowledge. However, after inevitably getting wiped on my first attempt, I kept trying to see if I could figure out this boss, and I quickly realised that it killed precisely those party members who were not in Zombie status. So I got the wrong message about how it worked -- I didn't figure out "this is simply coded as an instant-death attack, and as a general rule, all undead are immune to those" -- but I did figure out what I needed in order to work out a strategy for the boss.
I should note that every boss in Curse of Saria has a puzzle element; you're not expected to beat them on your first try, but need to figure out how they work and use that knowledge to defeat them. For example, there's one boss that consists of two characters, Craig and Brianna. Brianna can cast Haste and Regen on both, and if you try to fight normally, most of your turns will go to removing those statuses and healing the damage done by hasted Craig, and you just can't inflict enough damage to overcome the Regen. The solution is to wait until she casts Regen on both, then cast Zombie. Figuring this out was a lovely "aha" moment, because the puzzle was perfectly fair: you can scan monsters (and bosses) to see what statuses they are vulnerable to, and an earlier boss had used Zombie + Regen on you, so you know this is a mechanic that exists. There is a small flaw: you can't know before trying it whether Brianna has the ability to remove Zombie. But if you've gotten this far in the game at all, you should have gotten into a mindset of thinking up plausible strategies, and then trying them out.
So no, I don't see it as a "guide dang it", but as a puzzle where the game presents you with a seemingly impossible challenge, and you have to work out how it's possible after all. I really enjoyed solving it (for both the bosses I've described). And solving the puzzles didn't make the bosses trivial; it made them doable, you still had to win the fights, and they were extremely fun and satisfying.