Abbadon (not frakking Abby) is Warmaster of Chaos, Chosen of Chaos Undivided, a master warrior, tactician, and leader, and on top of it all he is, as our beloved Commissar Ciaphas Cain called him, a ten-thousand year old madman who's been marinating himself in the raw stuff of Chaos for most of that time.
We don't need a Champion or another leader to fight him, practically every single living warrior would be torn to ribbons by the Talon of Horus and effortlessly beheaded by the Daemon Sword Drach'nayen.
We don't need a warrior.
We need a frakking miracle.
Such as, but not limited to, *ahem* this is gonna be a doozy...
Random Warp-rift in the middle of the fight, either tearing Abbadon's very being it innumerable pieces, or bringing in a lost/previously dead Loyalist primarch to go,"You, sir, are a wanker.", and engage him in mortal combat.
The Emperor in his fully-healed glory being Abbadon's opponent, and him going,"Sorry, Empire to un-frak, gotta make this quick.", and commencing with the mind-bullets.
The Chaos Gods suddenly going, "We want to see how you fare without all this extra shit we gave you.", and stripping away his blessings.
In short, without some kind of playing-field leveler or Deus-ex Machina (where a God or some ultra powerful being breaks in and saves the day), anyone going against Abbadon in mortal combat, is frakked. Well, and truly, frakked.