Him not using Leviathan is alright, even though as a nerf it doesn't really matter at all. Regardless of if he uses Leviathan or not, Regulars should be ants in front of a higher dimensional existence when compared to Baam right now.
I hope the tournament is much more than just fights. Otherwise, it's going to be a real bother to see Baam struggle against people he can one-shot with his presence alone.
Another option is Rankers participating, but it should be Top 100 HRs and above (above really) if SIU wants to make the readers feel like Baam is in serious trouble. Then again, if he does indeed include Rankers, wouldn't that make the already sidelined Regs even more irrelevant? Since Rankers are like gods in front of Regs and all of that. Unless SIU finds some special rules/games to make this work. Regardless, having a tournament with both Regs and Rankers without destroying the lore he's built over the years would probably be extremely difficult to pull off.