Thinking about what Marcelee's stats might be like.
Martial: Her personal experiences and training the Penguin army would mean it's high, but she has no noticeable feats of tactical or strategic acumen. Probably not Paragon, then, not unless she did things in the parts of her life that we didn't see in her omake.
Intrigue: Ditto.
Administration: Below average to average for a human. If she learned things from the humans she met as an adult, she'll be average, but if her only education came from Sirens, I'd reckon below average. Sirens don't seem like the administrative type to me.
Learning: She's been everywhere, learned everything she could wherever she went, and beat Kairos Fateweaver in a game of wits. Yeah, pretty sure she's getting a Learning Paragon trait.
Piety: Multiple daemonic incursions, multiple encounters and fights with Exalted Greater Daemons, living on Avernus for tens of thousands of years...yeah, Paragon seems a safe bet here.
Diplomacy: She's made friends, but that's about it. Average for a human, potentially higher depending on experiences we haven't seen.
Combat: Paragon for abundantly obvious reasons.
Power: Confirmed Paragon IIRC.
Control: Confirmed Transcendent.
Eh if I can add my personal take on it.
Martial: Average at best possibly worse, she's never really been a commander. She can strategise her own fights fine, but not much bigger than that.
Intrigue: Fairly high, she is very good at hunting and tracking things without using her power due to sheer experience if nothing else.
Admin: Pretty below average yeah. She can manage her stuff enough not to loose anything of import and keep herself well fed and clothed, but that's due to her own abilities more than anything
Learning: Yeah high, partially due to learning so much Avernite stuff as you said, but also because Simon taught her a fair old amount before...you know. And she's very smart on her own.
Piety: Seems about right.
Diplo: I'd imagine the debuff to talking to non Avernites would be hellish, however she'd gain points talking to Avernites and a bigger one talking to People as most of them have some legends about the time she spent with them.
Combat: Also a few grandmasters i.e. Grandmaster of the Axe. Dunno about Paragons there. She also has one powerful artefact (her axe) and a powerful relic (her bow)
Power: Not confirmed paragon, but still high due to her being a powerful alpha.
Control: Yeah
So...you're saying she can beat most Exalted Greater Daemons as easily as she beat that one?
To answer this (sorry I just finished exams yesterday so I kinda crashed and slept so didn't respond) she didn't beat it easily despite how that could be interpreted. What she did was play its own ego and among other things which let her use "smart" tactics.
By which I meant she led the thing on a wild goose chase weakening it through her own powers and getting help from Guardians and Weapons to shoot the ****er. Then when she felt confident she could take it one v one she did. Though admit ably one of the big things was that she was able to get in a walloping attack right off the bat that severely weakened the Demon.
And due to its ego it didn't call for its own back up, because it refused to be humiliated by a mere human.
As for how she was able to perma kill it, well she was using a variety of true death techniques and it had bound itself too closely to the Avatar. While in most circumstances this makes sense (the avatar is a big power boost due to being such an effective conduit/shell that doesn't have any pesky creature left in it to resist, its basically perfect for a demon) in this case it meant that Heartslayer wasn't able to remove itself from the Avatar before she was able to incinerate it. Combined with the damage it had taken from her basically going straight for the nether ballz and well.
To note this did partially work because of Heartslayer being...well heartslayer, not due to the demon being weaker than another exalted, but because of the mentality of Slaaneshi demons (excessive ego ect.) One like An'ggrath or Khabanda would not have fallen for this and Marcey probably wouldn't have beaten them. She'd have weakened them, possibly severely but only the same level that Kairos was afraid would happen to him.
In this case the situation is a little different, she wouldn't be able to lead the Angyle in a weakening run about because its focused on Lin, however she is strong enough to survive against it in direct combat, at least for a bit. Where upon we and Avernus can provide her with support.