Only reason he won't die is the enemy just playing around.
You're saying that like "the big bad is too dumb/arroagant to decisively kill the good guys before they become a problem" isn't the default assumption rather than the exception in battle shonen.
First off the enemies seem absolutely intent on killing the mc and he should absolutely die here.
I'm not sure his death is even her priority. Through the bullshit-knight fight until its conclusion her reactions were more curiosity than hostility. (with still a lot of hostility and disdain, not contesting that) It feels more like Abyss is very curious about this weirdo that Eldora created and wants to understand its capacities and limits rather than outright squish him.
Killing him (or at least threatening to) being the obvious best way to evaluate said limits for someone with her mindset.
Eldora should also never be notified in this situation. The other guy has no known reason not to stop the girl, either killing her or taking her hostage.
Right. If we exclude bad writing, the other explanation would be that she actually doesn't care about Eldora coming as long as it's a bit delayed. Probably in part because she's a dragon (semi god ?) and nobody's going to stop her from flying away (she doesn't even need to transform ?) And if her objective is really to kill Dion she assumes she can finish him in a snap of her finger. Alternatively, she doesn't care that much about whether Dion lives or dies and her actual objective is to make Eldora suffer and inflicting grief, anger and a massive dose of paranoia is good enough for her.
in a story like this [fights] should at the very least be interesting
Subjectivity aside, isn't that something that can only be called once it's over ? Or at the very least, not after just one chapter ?