That's the thing though. We can't prove we can defeat Kil'jaden unless we're willing to summon him into Outlands and kicking his butt.
But that'd be practically suicide since Kil'jaden is no weakling. Even the Murderhobos of Azeroth could only banish him back into the Twisting Nether rather than kill him. And the Elves had to sacrifice their Immortalit just kill Archimonde. Hell, even heroes like Jaina *Miss I can submerge Orgrimmar under a tidal wave of my own making* and Thrall, arguably the strongest shaman alive could only slow Archimonde down. And Kil'jaden is arguably stronger than he is. He's definitely smarter.