Will's Advantages/Win Conditions
Will starts out with and advantage in power (1.3 times), speed (1.2 times), and lifting strength (Class 50 vs Superhuman) which would help slightly (I'll explain why it's not a massive deal in Danny's section). However, her overall experience and skill does exceed Danny's since she's been at this for at least 3 years and has actual martial arts skill and is comparable to Hay Lin, who has skill like an expert martial artist. This means Will holds the close combat advantage, along with her clothing which defends her instinctively.
As for actually causing harm to Danny? Normally, the Guardians can't touch intangible beings but they were capable of focusing their magic to attack the consciousness of someone possessing their team member so once she sees Danny can turn intangible, she can tune her magic to hit his consciousness. As for Danny's invisibility, Will is capable of sensing an objects energy and how potent it is, and has a danger sense so she shouldn't be caught too off guard by this and she has her own invisibility.
For her other hax, Will's not known for using it even against serious opponents but what she does utilize is her teleportation (which is lightspeed) so this can help her dodge Danny more effectively. I think another important factor is Will's better range (Kilometers) and Stamina compared to Danny. She's superior to someone that has more energy than 1000 suns and is vastly superior to Taranee's old form which survived 2 weeks in a cage that was draining her energy.
Danny's Advantages/Win Conditions
While Danny is slightly outmatched in power, speed, and lifting strength (more so lifting strength), his reactive evolution will buff him to her level in no time. And while he's outmatched in terms of experience and skill, Danny has fought people with more expensive than Will, and he can somewhat counter her skill with his body control to some degree. He also has ways to amp his strength with his ectoplasmic energy and rage power which will allow him to overpower her.
His regeneration will allow him to hold up against her attacks, and if he lands a possession, it's over. Like Will, Danny has the ability to create forcefields that block and reflect attacks. His intangibility can also help early on since Will doesn't naturally effect intangible beings with her magic unless she tunes it to. His ectoplasmic energy attacks do ignore durability though I doubt he'll try and vaporize some girl on a molecular level right off the bat.
As for other things, Danny's range and stamina aren't as good as Will's but he can still fight for hours and can reach hundreds of meters with his attacks and Will doesn't abuse her range all the time so he can keep up with her. Not to mention, Danny's ghost rays are light speed like Will's teleportation so unless she is away enough she's going to get hit.
My Vote
I'm going to vote Will (Though I wanted her to take this L)
While she's not going to use her more potent hax as it's out of character. Her overall skill advantage tied with her various senses, teleportation, range and massive stamina advantage would allow her to match and outlast Danny with her main issues being his Possession and Ghost Rays but considering her senses and own light speed movement through Teletransportation, she should be fine.
Both can dodge, block and reflect attacks but the problem is, Will's will simply absorb and grow stronger from her own magic being thrown back at her, while Danny's attacks will be sent back at him with twice the force. In the end, after a hard fight, Will should come out on top.
