Finn's archenemy, or at least the one Disney acknowledged as such, Phasma was a relatively tough sadist, though in the end not so effective. Whom can she beat, and who would mop the floor with her?
Can: Rose Tico
Rose might have grown stronger, having a blaster, a tazer and piloting skills, but on foot she is still not an one-woman army. None of her attacks can do anything against Phasma's armor, and Phasma can shoot her dead quickly.
Can't: Pierre
It's the battle of the warriors in thick armor, each wielding a powerful weapon: Phasma her spear, Pierre his sword. Aaand Phasma is done for. Pierre's armor is even tougher than hers, he is much faster, and while his beams of light won't do much, Alexandros can get through Phasma's armor and cut her down.
Can: Fake Finn
Finally, a way Phasma can get her revenge! So, Fake Finn and Phasma are near-equals in skills. Fake Finn will last a bit in close combat, and he will also manage to land at least one shot. However, Phasma's armor can stop his attacks easily and she has the advantage.
Can't: The Loyalist
Nines has got a massive power-up as the Loyalist. He wields a Riot Baton, which can hurt Phasma, with fearsome skill and speed, and while his blaster is just an annoyance to Phasma, he can cast Hellfire which is far more of a threat.
Can: Matt Engarde
Yes, he is much smarter, and yes, probably his spear can do something. Probably. But between the cowards who rely on arsenal and other people, Phasma stands more of a chance. Her arsenal is superior to his, as both wield spears, but she has a blaster and not thrown knives and a fan. Also, while the Nickel Samurai armor can take hits, and while Matt does stand a little chance, Phasma's armor can block the thrown knives and the broken glass shards he can throw at her, then shoot him dead a few times.
Can't: Din Djarin
This case is similar to Pierre's. Din has better mobility due to his jetpack, even better armor, Beskar, which can block even Phasma's spear, he has a flamethrower to distract her, can fire missiles, and above all else, has the Darksaber which can damage Phasma. In short, Phasma is outclassed.
Can: Nessie
Nessie is a case where she is not very powerful, but she is surrounded by many strong guards. Her telepathy is only good for commanding, since she can't reason with Phasma. While Phasma's spear is not a stake, it can stun Nessie enough for the real fatal weapon, the blaster.
Can't: The Nutty Poet
Phasma only wins in durability this one. The problem is that, unless they are in an open field and she knows to melt the damn ice walls quickly, and doesn't lose time with thrown snowballs, the Poet will make a weapon, mostly explosives, that can get through her armor and mess her up.
Can: Emil Kudos
Sure, there is no chance she will reach him, because Emil has created the Force Hunters that can oneshot her, but he himself is not so powerful. He can set his foes on fire, and fire poisonous needles and tranquilizer darts. None of which can penetrate Phasma's armor. On the other hand, her spear and blaster can easily take him down, especially since Kudos is not prepared for anyone who can fight back.
Can't: Finn
I just couldn't avoid mentioning him while talking about Phasma. She was once a threat to him, but considering he beat her in the Supermacy, and grew much, much stronger ever since, he currently can beat her in the very first attack he can make, be it a well-timed Power Slash, a Microwave Beam to the helmet or a sticky detonator.