As a fan of both Star Wars and RWBY, I'm gonna rank this as evenly as possible. First i'll talk about each combantant than go into a comparison of:
Weapo
Abilites
Armour
Experience
And an actual fight between the two
[THIS IS A RESPONSE TO THE WHOLE MERCURY BLACK THING. NOT RELATED TO RUBY ROSE VS BOBA FETT SKIP THIS PARAGRAPH IF YOU WANT MY ANAYLISIS] Okay, first of all POWER RANKINGS DON'T MEAN NOTHING. They don't mean anything and shouldn't. The whole Mercury being sigificantly worse then Boba is dumb in the simplest way. They're both trained killers but to say Boba Fett is better than Mercury at his own game, is stupid to say the least. Mercury's whole fighting style (minus his dust cyclone) is h2h. Boba is trained using weapons. That is Boba Fett's game. If you are talking specifically about h2h Mercury would beat Boba to shreds. And if you use your so precious power rankings, Mercury's metal legs > Boba's clone legs. Mercury's aura > Boba's armour. Mercury's 95% h2h fighting style > Boba's 30% fighting style. Now that I've gotten that out of the way, Ruby vs. Fett.
First Boba Fett. A clone of Jango Fett one of the best assassins in the Clone Era. Boba was trained by his dad and numerous other assassins. He has a really strong offense with his missiles, flamethrower and signature gun the EE-3. His jetpack also gives him a stronger air advantage than Ruby. He has extensive training in h2h combat and is a very strategic mind. Boba Fett has killed mass amounts of Jedi's with not only his weapons but also h2h. He has defeated large amounts of clone/storm troopers, civilians and survived battle with Darth Vader twice. He drives the Slave-1 a space ship previously used by his father that is extremly powerful and gets +1 point for looking badass.
Boba Fett is known as the deadliest bounty hunter in the galaxy of the Star Wars Universe at his current era.
Ruby is the leader of Team RWBY and the youngest known student at Beacon academy. Her semblance is speed were she turns into many rose petals to either escape from her opponent or launch herself to them. Her weapon is Crescent Rose, a scythe that can turn into a sniper rifle. She proved to have a strategic mind in her fight with Roman Torchwick, a low-life criminal who was currently sporting a giant milatary mech. Assisted by her team she listed numerous team attacks for her friends to defeat Torchwick. She has defeated numerous Grimm (not including the dragon grimm because "Ruby has dust ammunition but does not have the Silver Eyes.", numerous white fang members, numerous atlas military and made Cinder Fall (the strongest character in RWBY at the time) to flee from battle.
Ruby is a star student with many achievemants but still has a long way to go before she is a true huntress.
This battle at first glance seems very one sided. But by both parties. At first I personally thought this would instantly go to Ruby as her aura and semblance gives her a clear advantage over Boba. Crescent Rose is seems much deadlier than EE-3 because of its wide range of offense. Crescent Rose is a scythe that casually cuts Grimm in half and can turn into a sniper rifle capable of launching Ruby in the air like a pogo stick. EE-3 is a efficient plasma rifle that can kill storm troopers in a single shot. It doesn't have as much variety as Crescent Rose so 1 point goes to Ruby.
Now abilities. Boba Fett is a clone of Jango Fett and has supereme combat knowledge. Ever since a young age he was able to handle himself in combat an is regarded as a killer. He is very fit and demonstrates extreme stamina in his fights with Jedi and large amounts of soldiers. Boba Fett is also very strong as he can easily throw opponents much bigger than him around like toys. Not necessarily giant opps. but still heavy ones. Ruby Rose has the semblance of speed. I'm not sure if the rules dictate she is not allowed to use it "All speeds are equalized." but if she is she will be much faster than Fett. She also has the ability of aura. All huntsmen and huntress' possess this ability. It is described as a forcefield by Jaune Arc, a young huntsmen in training. It allows you to have a wall of defence around you at all times when it is activated. It also allows you to explode an opponent (only demonstrated on Grimm so will not be included). Ruby also proves to be very strong as she hung a Nevermore (Giant bird) by her scythe and ran up a cliff, decapitating it. Ruby also is very agile and has loads of stamina never getting tired during any of her shown fights. Again I believe Ruby is the succeeds in this category.Her feats of strength, stamina and agility are much more impressive than Boba Fett's.
In armour Ruby doesn't have much going for her. She was her cloak which has proved to be more oh a hazard then anything, almost getting her killed by a Nevermore. Besides that she has mostly regular clothes, nothing really protecting her from a gun shot to the chest. However, her aura protects her from any weapon (except Adam Taurus' sword) I don't think a light saber would be able to cut through Ruby because of her aura and a light saber isn't regarded as one of Boba Fett's trade mark weapons so I won't count it. Just like how Ruby using Penny's sword doesn't automatically mean she gets to use it in the Boba Fett fight. Ruby not having armour lets her use her agility and speed instead of her having a protection boost. Boba Fett has mandaloria armour a durasteel alloy that can sustain multiple gun shots. The armour also comes with a jetpack with a missle, a hidden flamethrower in the wrist and a whipcord thrower. The armour is famously worn by human warriors of Mandalore. Most famously worn by Boba Fett and his father Jango Fett. The armour also has a small portable shield and projectile buzzsaws. Comparing the two armours the clear winner is Boba Fett. I do think Ruby's aura is stronger than the bounty hunter's armour, Boba's armour has many hidden features that includes offense and defence. And because of that I give the edge to Boba Fett. In experience both combatants have lots of battle history. Ever since Boba Fett was born he was trained by his father to be an assassin. He has extensive training in combats like h2h, sharpshooting and possibily piloting. Boba is supposably 32 BBY giving him at least 20 years fighting. Ruby Rose is 15 (possibly 16 in the Volume 4 Trailer) Boba Fett's years of experience out weigh Ruby's age. However. there lives are very different. Ruby is the leader of Team RWBY and fights off Grimm for a living. Her job is to protect the world from harm. Boba is leads a syndicate of Bounty Hunters who preform less then admirable tasks to earn money such as killings of Jedi and political figures. I do believe that Boba Fett has more experience that Rose in just time alone. The fact that his skills have existed before his opponent is a huge advantage. Finally. both competitors have 2 points on their name. But now it's a fight scenario. The fight won't take place in eithers world but just in a plain field of grass. Now in general via the fight rules it gives Boba Fett a clear advantage: Saying "EU Boba" also saying each others speed is equivalent, basically getting rid of Ruby's semblace. However, i'm gonna let her use it without Boba Fett becoming a speed demon because thats just fair. It also gives Ruby dust ammo but no silver eyes. If she had silver eyes she would instanly win but it is fair to get rid of them in return for the special ammo. Dust is basically magic manipulation. It lets you use a artifical type of magic (E.g. Mercury's Cyclone of Bullets or Flynt Coals Trumpet) to your advantage. Now in a fight I believe it would be a long valiant fight with Boba Fett retreating to the air only for Ruby to use her semblace as she did in the Volume 4 trailer to get behind him in the air and destroy his jetpack. I also believe that in the fight Boba would disarm Ruby from Crescent Rose multiple times and deplete her aura whilst she runs for her weapon. The final result I believe would have Boba Fett being the loser. The lack of armour on Boba's neck, torso sides, arms and legs would ultimality make Boba end up losing. Those areas are the prime area for a Scythe to attack. Also with the addition of dust ammo I have no doubt that Ruby would easily out class Boba in range. Boba being better with his EE-3 isn't as important as Ruby casually blowing Fett away with a wind bullet or burn him with a fire one. At first glance the fight seems pretty one-sided, but when you look at it from another angle it is extremely close. I have no doubt that this is a huge toss up between two characters from seperate universes but Boba Fett has reached his glass ceiling. Ruby isn't even close to it yet which is why I believe she would win this encounter.