VOTE FOR WHO WOULD WIN AND WHY. VOTE HERE.
Thalia, because she can summon lightning.
Depends on on if Jack is apoat of the fight or not. If not it’s a win for Thalia. Magnus is no match to Sam when they fight and Thalia is in a complete different league of spear fighting then her, with better equipment ( knives, spear, Aeges, and hunter bow) and better powers for combat. If jack was in the fight really depends. We do see characters actually duel and fending off him. Could Thalia fight off jack until she can get a good hit on magnus. Probably. Another thing is Aegis. Could she just scare jack and magnus and go for the kill? I would really think so. So yea I would go with Thalia
Like Housedunn said, if Jack is in the fight Magnus has a slight advantage. But Thalia can summon lightning, and is way better at fighting than Magnus. Plus, she has Aegis. And Thalia's a Hunter of Artemis, which means she's been fighting monsters for a while so she's really good at it. A talking magic sword who can't sing pop songs isn't really a great defense against Thalia.