Scooby's toon force and nanite Scooby Snacks would make him very unpredictable and help him take what ever Ruby can dish out to out last her until her aura eventually drains away and she's vulnerable to being taken out.
Scooby's Nanite powers would include invisibility, force fields that allow him to bounce into enemies like a giant superball, he can turn into two types of giant forms (One a giant robot dog and the other a giant tentacle monster), he gains the power to stretch, he can become a living magnet (which can allow him to rip away Ruby's weapon), super strength, a sonic howl, shape shifting, and multiplication.
I assume that Super Speed and Hyper Speed aren't going to be used in this match since speed is equalized, so I'm excluding them.
And yes, these powers only last a minute or two before going away, but he can easily just eat another nanite scooby snack and gain another super power. Plus Ruby wouldn't be able to eat them and gain their powers herself since the nanites only affect animals.
And assuming Scooby can bring the Yellow Lantern Ring; while he doesn't have much experience using it, it would allow him options to defend himself with additional force fields and improve his manueverability since he gains flight.
Her only hope would be to petrify Scooby with her silver eyes, but not only would that be difficult since she's yet to fully control it (making it unreliable and a lucky shot at most) but the second he see's she's starting to freak out and glow, he's likely gonna run away in fear and avoid getting hit. Once that happens, she'll be unconscious, thus giving Scooby an oppurtunity to finish her off.
Sure Ruby is a much more experienced honed fighter with a more practical weapon/abilities better suited for combat, but Scooby is still very tricky to deal with and is unlike anything she's used to fighting, especially since he's not a direct fighter and would prefer to keep his distance or keep stalling for time with his powers/abilities, which can end up wasting her ammo and aura.
So yeah; a vote for Scoob
