REMNANT: Buffy, the Vampire Slayer meets Batman and the Red Rider - Chapter 1.

A dangerous sorcerer named Felix Faust has come to Sunnydale, intent on raising an ancient demon to subjugate all of humankind. To succeed, he must perform a powerful curse to displace his enemies, transporting them from one time and place and exchanging them with others from that region.
Buffy Summers, the 16-year old Vampire Slayer, has never gone up against a wizard as powerful as Faust. Not to mention his allies, which include Buffy's old enemy, Ethan Rayne, and the fear-producing Scarecrow, along with a deadly witch from the past. To stop Faust from unleashing Hell on earth, she'll need all the help she can get.
When Buffy and her friends partner with the mysterious superhero detective, Batman, and a legendary heroine from the past called the Red Rider, she finds the exact help that she needs. But while they're scattered to another time with no way to return, even Buffy's new allies struggle to defeat Faust's forces in time to prevent an apocalypse and find their way back home.