Can it be SNEK TIEM?
Well, it's a bit different from real invincibility. You see, the defense ring provides a flat bonus to defense which is about the equivalent of an armor upgrade- and the fact that this version of the game doesn't have pity damage works both ways.I can't believe you get access to damage immunity this early in the game.
I think the main weakness of this version of Lagoon is that it is not only possible to outgrind pretty much all of the bosses, it may actually be necessary, because all of the bosses do body contact damage. In Ys I, II, and IV (Mask of the Sun and Dawn of Ys versions), most bosses either didn't do contact damage, or if they did, there was a way to attack them that didn't expose Adol to that contact damage, and MP isn't plentiful enough in this game to burn down bosses purely with magic. So while this version does play better in the general, it's not without its own major flaws.