Finally the Red Seal is broken it was very tough but Mazenda saved me with Full Cura Note that most of the fight was made by auto hitting and Silence on all enemies was great until it wore off when they almost destroyed me.
I don't recall if I followed Commander's advice and waited to battle this boss until later in the game. However, I did find that many of the boss monsters can be hit with ailments that really help you (paralyze, which only lasted a turn, was good, poison was of a little help, and debuffs were very good).
TDS (Transformed Dark Spirit) is dealing around 200 damage unless I debuff the Attack but when it wears off I'm dead (I completed some new areas and it took me 40 minutes to go through Arctic Plateau puzzle by myself which is a clever puzzle imo. Hardest part was to remember where each person was and the fact that I tried to go out the wrong way. And when I realized that I need to have one more person out before everyone can get to safety.. Dang. )
Now I'm in the Tower of Trials and that's the place that will take more time due to having treasures. And stairs everywhere. (37 Dragon Pearls and still missing two key items but I'll find them sooner or later)
I can one shot enemies there with Lufa but priority is chests and as you said earlier to not overgrind, I try not to but xp gain right now is high enough. I just have to find the rest of items +9 secret equipment (At least I know the location of one but as I mentioned earlier it kills me because I have no high hp. Speed isn't a real issue due to Quickra spell.
I've got some more questions :
Are bosses immune to Death like in many other games?
Where is the most challenging boss (you said Dragonlord isn't the toughest boss so I'm asking that. )
I'll want to pay a visit to Dungeon of Trials as well because I wasn't there for a long time.
Besides that I cleaned the Arena from all objectives needed to complete the post-game. Note that I don't see anything bad in devs making Arena a must to do place because it doesn't have a big challenge at that point (well at least if the party is just after Labirynth of Fire).
At least I know that main game is best cleared without doing job changing because the xp is way lower than the Post-game (started Changing jobs after being defeated in Dark Forest. )
And I want to beat Yorozuya RPG too but that can wait.
I believe the bosses are immune to instant death, but not immune to many status ailments.
My favorite boss is in the Tower of Mirrors, if I remember correctly. More fun than the DragonLord, and if you aren't thoughtful, you will lose. Not necessarily difficult, but more interesting. Just about the only boss battle I lost and needed a second/third try to win. I doubt you will overgrind if you just do everything, as that makes you quite strong. With party members having the FullCura, as you know, you can stand quite a bit of pain. The DragonLord is a standard just chop away/FullCura and you should have no problem.
So have fun with the remaining battles, and don't despair in some of the tough puzzles.