For me it began to look bad when he killed the undead dragon himself with almost no effort, but it was still good... For me they killed it when Elise appeared and the mangaka changed Louise and Ina for the sake of the harem plot!!!
After that the plot lost its North and any action or romance look forced and boring...
(just read this) and yes, when he killed the undead dragon by himself was too OP, even for an isekai manga but it was still ok because... welp, it wasn't that terrible and the plot was still there. Definitely the fatality was when they made the sacrifice for the sake of the harem plot.
Sadly I stopped reading it since then, so... ┑( ̄Д  ̄)┍
The plot of this manga was killed