@Solwern I have several issues with the bossfight you shared.
Graphics are the first one. Everything is so blue it looks ugly. The background, the Battle status window, the aqua skills (even though they are blue naturally, so there's not much to do there... But you understand my point I hope). Also, if you have three characters, I don't see a point of having an empty member slot (can be fixed by tweaking the YEP_BattleStatus's width).
Then from what I have seen, the game doesn't really have strategy. You stacked up barriers, buffed behind them and slowly took down the boss bit by bit. I'm not sure if you saw Yanfly's Unexpected expected battle on youtube featuring "Harold". If not, maybe you should look at it. But the point is, if a boss is uninteresting, it's pointless to have a long fight.
The skill arsenal of everyone, including the boss, is very limited. The boss just casts one aqua attack, only when his hp goes under 50%, he casts some strange looking auras.
Instead of casting his aqua attack, he could have had normal attacks and it would achieve the same results.
Except for the change in BGM I saw nothing that would even suggest a bossfight.
I cannot judge too much more, since I haven't actually played it. But this is what I see in the video.
I would sugest the following.
Spread skill arsenals of at least the boss, because his one attack is seriously pitiful. But the fact that your skills pretty much force the game strategy isn't ideal either.
Actually engage the player. Animated background, dialogs, plot twists... So much to do with the bossfight to make it more lively.