3.5 was edition I played the most. It was a reason why I quit RPGs for nearly a decade because I hated it so much.
Every time I see another meme about how amazing 3.5 Tarrasque is, I remmember how amogn actual 3.5 players Tarrasque was the biggest joke. It was always brought up as definite proof designers have no idea how to make good monster. It was laughably easy to beat. A wizard could casually solo it, the same abilities people now miss in 3.5 amounted to ribbons. It was a laughingstock, forums had 100+ pages discussions how to fix it and general consensus was it';s beyond saving. It was first proof in 3.5 if you cannot use magic you're only good to roll over and die.
I honestly don't know if everyone claiming 3.5 Tarrasque is such a horrifying monster are trying to rewrite history or unintentionally proving what a broken, unplayable pile of garbage 3.5 was, if it's biggest punching bag is actually dangerous in a different, better designed game.
About 14-15 years ago, I was playing in a 16th level game where the DM did NOT know how to challenge us. He put us against an astral behemoth with double hit points and our fighter soloed it in one round, dealing out a whopping 2,500ish points of damage in 7 attacks. One of the toughest monsters in the game, with double hit points, and the rest of the party didn't even get to act.
Later in that game, we abused gate spells to crash rocks into the Abyss at 80% the speed of light.
3.5 is ridiculous.
I know this is a long shot, but can you remember how they managed this? I've played pathfinder and this still seems like ten times more than what a well-optimised could do!
Ah, now this just sounds like the DM didn't know how to say no to your crazy ideas that don't fit into the rules!