Did you ever play a game where it doesn't increase in difficulty the more you advance in it?
It's the same here. The game slowly introduced you to new mechanics. You're not meant to zerg and braindead the bosses. You're meant to learn their mechanics. Most ( If not every ) boss in the game starts the encounter with an introduction to their mechanics, which should give you an insight into what they can do and how to avoid it.
As a mentor and a casual player. The problem isn't with the bosses' mechanics but rather it's with a certain portion of the players who don't read the chat, don't know the basic mechanics, and are not willing to cooperate and learn. Those are what makes a trial boss hard, not the boss itself.