Numbers are not applicable here, especially since it hasn’t been confirmed what some characters’ levels are like Morden’s and Julian’s. Morden’s level may not be comparable to the player’s, especially since he kept his memory and thus still has his prior experience using magic. For all we know, Morden’s level could be much higher or much lower than ours.
I think we should look at abilities instead. Morden with his unmastered Death Curse tied with Julian’s experienced Glass Curse. The Atlantean may be at the max level for the player, but it still doesn’t have any lost/ancient abilities, meaning it must be significantly weaker than what your average max-level player would look like.
If you look on YouTube, there are many examples of level 125 players being able to defeat Unholy Beast Atlanteans by themselves, if they use the right strategy. So I’m sure Julian’s Glass Curse is more than enough to deal with a base-magic/fs-wielding max-level monster. He could hover extremely high in the air over it and rain down extremely sharp barrages of glass. I do not believe the Atlantean could do anything about this strategy. It could send blasts of magic or try to reach his height, but because of the distance, Julian would have more than enough time to react and dodge by flying away. In conclusion, Julian stomps the Unholy Beast Atlantean with extreme ease (still not as much ease as defeating the player, though).
As for the Legendary Beast Atlantean, Julian would not survive. Durza wiped out 80% of all life and he was still losing to Theos, who is level 1500 at most. So anyone over 100,000 could definitely blow up the planet on a whim. No level of flight or curse mastery would allow Julian to escape the size of a single attack.
Numbers are still helpful for determining the Canonical strength of the unholy beast, thanks to their level we do know that they do have similar stats to the most likely max level player barring possible stats from armor and accessories.
So long as max level players aren’t required to use lost/ancient abilities at max level and it isn’t confirmed by vetex that canonically the player character used them by max level than the player character would still be capable of beating Julian without ancient or lost abilities and thus so would the unholy beast. As far as I’m aware we don’t have any reason to assume either of those(required ancient/lost abilities or vetex’s confirmation the player character used them) will be the case. Personally since we don’t have that confirmation I don’t think those abilities should weigh too heavily in this argument, you still can if you want, but for my own argument I’m not gonna take it too much into consideration.
Honestly pretty sure the only reason we are even capable of it is just gameplay reasons, I don’t think the player is canonically beating people 5x stronger than them even with good strategy, or has even actually gone to the dark sea yet. However lets say for the sake of the argument that the player is canonically capable of beating people 5x stronger than them then chances are the same level 125 player could canonically beat Julian under the same conditions, where he’s not in his guaranteed win cutscene and isn’t sneak attacking us but rather us sneak attacking him. Obviously its totally possible that he still wipes us even with strategy and all the abilities we have at our disposal against unholy beasts, but since we can’t actually fight him we don’t really know. You don’t have to tell me this is a weak argument, I know but thats just if I use a steelman, again its most likely not canon that the player character could beat or even met unholy beasts in the first place, especially since they’re already at our max level.
This goes both ways, the fastest attack we’ve seen Julian use is probably when he launched himself like a missile at us which the player character was able to react to and start moving they just weren’t fast enough to get away. The unholy beast probably is, but even if it isn’t it can definitely tank it since the player character who is much weaker could survive it even if just barely. After either dodging or tanking the unholy beast is gonna hit Julian with a pretty mean spell if not multiple before he gets back up which given their insane damage could be fatal depending on how many he’s hit by. If Julian is even capable of shooting glass everywhere at the ground all at the same time then the Unholy Beast is probably just gonna end up in the water where Julian won’t be able to do much from up in the sky, so that strategy only really works if they’re somewhere with little water which would be kinda hard to come across in the war seas plus I don’t think Atlanteans can survive very well outside the dark sea in the first place so it might be a little unfair if it happens outside of it.
This isn’t to say I’m certain Julian loses, but given all this even ignoring everything except the battle strategy that either Julian or the Unholy Beast employ the battle seems a decent bit closer than just Julian stomps, and simply staying doesn’t seem like a very viable strategy unless there’s just straight up no water around in which case again, Unholy Beast is probably dead anyways. I would still agree that even if the Unholy beast were to have an advantage on damage and I think could still land a few hits, Julian still has the advantage thanks to the curse making him a pretty hard mosquito to swat.