Golem Twins is the hardest fight in CT and absolutely obliterated me as a kid.
They copy the elements you use on them and retaliate with these huge AOEs, or they'll just stand there casting HP-halving iron balls if you do nothing. You can get these elemental armors that absorb damage, then just use that element and win the fight easily, but I didn't know that as a kid. All those elemental armors are pretty well-hidden. And if you didn't find them already, backtracking out of rhe Ocean Palace takes a while.
I actually suggest doing the fight without the armors because it's totally winnable and one of the few actual challenges the game offers.
Giga Gaia is probably the 2nd-toughest for me for similar reasons.
I never used Robo much and always thought he was the sole character in the game who was a bit of a step down from the others (in a game with incredible character balance). He's basically the jack of all trades type who uses Shadow-type damage until you get the guy who's actually good at it. Was never sure why there were two Water elemental characters instead of making Marle "Ice" either.
My main party later on is Crono/Marle/Ayla. Pretty much an invincible party. Can go Crono Confuse / others Cube Toss for huge single target damage, or have Marle heal while others use Volt Bite. In the lategame Volt Bite and Cube Toss are two of the strongest moves.