That the most important tool is your mind:
I've played alot of rpgs and this is the most common thing I think of whenever I lose on a boss I can't beat, I usually take a step back and think of what I can do differently, what resources, spells, techniques, etc. Will give me a chance at winning.
It became apparent to me that a little adjustment to approaching a certain part of a game that seems impossible, can go a long way, and the game that encourages this mindset the most from what I've played is Valkyrie Profile.
As alot of bosses and enemies in that game are damage sponges with alot of hp that do absurd damage, to the point you start to think it's intentional, as the game gives you lots of tools to play around this, it's how you decide to do it is what's important.