the new trial is too hard

[Zaft]

My biggest issue here is the confusion
I don't know if I'm doing it wrong, if others are doing it wrong
like everyone says different stuff and that confuses me big time

You need two people on cannon/tether duty. You need one person on tower duty. You need one tank on add duty. You need (at least) one healer.

Everyone else attacks the dragon and doesn't stand in things. That's it.

Hasn't been in my experience. I have been one-shotted by several other things in this fight, like exploding berthas.

Speaking of Vishap though, his feet do way too much damage, considering the strategy seems to be "pile the mobs under him to get blasted by the cannons"

First off, the only attacks that hit you are attacks that can be avoided. If the dragon jumps, you didn't tether him. If the turtle stomps, you didn't stun him. If the Berthas explode, you stood in them while they were being destroyed.

Second, piling under him means piling under his BELLY, not his feet. His belly will not damage you, nor will his tail. Just don't move near the feet.