9spaceking would definitely beat seldiora in a debate.

Statistics: If you look at 9spaceking's winrate, it is 60%, a reasonable amount, However, seldiora has lost more than 80% of his debates. Even against a random debater, seldiora would only have 20% of winning. But combined with 9spaceking's experience with over 300 debates under his belt, seldiora has no chance of winning. It's just simple statistics, is all.

Topics: If you look at the topics taken by seldiora, he concedes a vast majority of debates, thus reasoning that he would take the opposing side to 9spaceking's usual forte. However, he would lose motivation as the opposing side is not what he usually is used to. Because he would have to research the new side ahead of time, 9spaceking could easily anticipate this information and the research would have to be nearly flawless, otherwise 9sk could poke holes in seldiora's arguments. Due to cognitive bias, seldiora would have a tough time battling 9spaceking who would be arguing his usual side of his stance. 

self-contradictory: Seldiora seems to be pressured under more serious debates. But even in his best debate on DART (https://www.debateart.com/debates/2264-tournament-r1-seldiora-vs-supadudz), he presented three points to help con, which even Whiteflame discouraged. Look in contrary to 9sk's best piece of writing, about how smoking should be banned. (https://www.deviantart.com/sel-diora/art/Puff-of-Death-458177289 -- I know it's my username but I changed it from long ago) Though a controversial topic, he notably pointed out the errors in people's thinking in "their body their choice", and invoked both feeling of distraught when smoking, along with environmental concerns. On the other hand, seldiora has failed to negate a similar resolution of "your body your choice" contention in Abortion, losing severely against Ragnar. Seldiora's research on the opposing side is extremely poor in comparison to 9sk. Take a look at this debate about banning assault weapons: https://www.debateart.com/debates/2231-resolved-citizens-shouldn-t-be-allowed-to-have-assault-weapons, seldiora only listed a few bare bone facts and only tied against his opponent. Looking at 9sk's research in comparison, he thoroughly analyzed countless areas and experts, listing a lot of convincing examples why gun control policies are bad, [https://www.deviantart.com/sel-diora/art/The-Ineffectiveness-of-Gun-Control-Policies-592871880] which would completely and utterly defeat seldiora's argument in this debate. 

Perhaps the best example of exactly how this would play out is https://gaymarriagedebate.jimdofree.com/, where pro is 9sk and con is a seldiora way of debating. Despite trying to push the Burden of Proof onto pro, con still established a case which slowly was tore down with illogical reasoning and numerous fallacies. In the end, pro upheld the idea that gay marriage should be implemented. Indeed, if you look at this similar debate which also concerned Incestuous marriage, (https://www.debateart.com/debates/2227-support-same-sex-marriage-endorse-incestuous-marriage-just-the-same) despite facing someone with similar amount of debates and winrate, seldiora still lost immensely, showing his lack of ability to argue the opposite in what he believes in.

Conclusion: Seldiora is self-contradictory and helps his opponent countless times through debates. He has far fewer debates than 9sk under his belt on DART, and has failed to win against even lower ranked debaters. Just how can he possibly win against 9sk?? He has no chance. 9sk will win beyond the shadow of a doubt.