not even a contest. healing factor+4th wall for deadpool. Domino's luck+experience would kill them all. honestly, its nonsense framing it as deadpool vs joker, when Domino would be the main killer.
having now watched it. I would have changed a few things. Harley standing up to Domino for more then like 1 second of a headshot is silly. Would have started with Harley vs Deadpool.
And then taken advantage of Harley better. theres a few things more casuals don't know about her.
First, modern Harley pretty much IS DC's Deadpool. Also totally 4th wall breaking.
second, she is a metahuman. its subtle and rarely overtly referenced, but it is canon she is. Arguably cartoon physics. its not certain if that is what they are. Or, just super strength, durability, and some precog. her powers make her crazy as well. they are tied, in some way. the more crazy, the stronger they are. this predates her relation to Joker, and the new 52 acid bath as well. Her hallucinations can provide knowledge she should not have. the giant mallet is not just a comic/cartoon fiat. its either the cartoon physics or superstrength that lets her do it. others can't swing it around like her, and its been commented on a number of times, it shouldn't be possible.
she ALSO, has Poison Ivy has her lesbian bestfriend/lover. Poison Ivy has done alot of stuff to her. Shes immune to pretty much all toxins, venoms and poisons, etc.
none of this would stop a bullet to the head from Domino though. But a Deadpool vs Harley fight without her giant mallet is a mistake. And I think having this kind of thing without establishing what Harley can actually do, is a misstep.