Team Big Gunz
Team Toxic Arrow
Starrk >>>> Mayuri
Matsumoto >>>> Uryu
Big Guns wins.
I'm surprised Team Toxic Arrow made it this far (especially as I figured Komamura's Bankai would be perfect for smashing Mayuri's Bankai to smithereens before he even got the chance to release the poison), but Team Big Gunz would definitely beat them in this final round.
Interestingly enough, we've already had Stark VS Mayuri in the regular tournament, which Stark won (with quite a big margin, I believe). Matsumoto VS Ishida would be quite an interesting match to see, but really, they don't even matter in this fight. It all comes down to Stark and Mayuri and unfortunately, Stark is so far ahead of Mayuri in every single way except intelligence, that Mayuri is boned. The problem for Mayuri that his intelligence, his gadgets and his abilities do not make up for the incredible advantage that Stark has in every aspect of combat. Not without prep time, which he won't have in this fight.
Let's look at Stark's stats. Attack power? Capable of shooting an insane amount of Ceros and has the wolves, which are highly dangerous and very hard to deal with. Defense/endurance? Took a hit from Love's Tengumaru to the head with barely a scratch; was stabbed by Katen Kyoukotsu's big-ass Dao-ish blades in the back and was still capable of fighting and very well, at that. Speed? Appeared right between Kenpachi and Ichigo, snatched Orihime right out from under their noses and got out before their swords even had time to connect; had no problem with dealing with Kyouraku as far as speed was concerned, either. Skill/intelligence? Saw through Kyouraku's fighting style, figured out Sougyou no Kotowari's ability in three hits and understood Iro Oni's rules in one; showed impressive sword-skill.
It's almost uncomparable to Mayuri, who, as far as outright combat-skills are concerned, shown far inferior abilities. Even against Ishida back in the Soul Society Arc, he resorted to using Nemu in order to get a hit in with Ashisogi Jizou. Granted, he still could've beaten Ishida without her, but I don't think is a good indication for Mayuri when it comes to his performance in a straight fight.
This is how I see the battle playing out.
With the speed Stark has shown, I think he could kill Mayuri without releasing if he's serious from the start. However, Stark being generally lazy, he won't do that. Any attempt from Mayuri to attack will be pointless, though. Stark is more than skilled enough to deflect any hits with his sword without releasing and he'll definitely avoid being hit by Ashisogi Jizou. In fact, I'm not even sure whether the blade would pierce Stark's hierro, which seems very strong. Maybe if Mayuri had more time to analyse Stark, he would be capable of finding a weakness, but no matter how lazy he is, Stark will move on the offensive before that. Once that happens, he'll tear Mayuri up. Perhaps with that special medicine, Mayuri will be able to recover some of his wounds, but he'll definitely be forced to resort to Bankai if he wants to get anywhere.
Against Konjiki Ashisogi Jizou, Stark will be in trouble. However, unlike most opponents, Stark has both the intelligence and the speed necessary to deal with the poison. The second someone like Stark sees an unknown gas, he'll get the hell out of the way and with his speed, he'll be able to put enough distance between himself and Mayuri to avoid getting poisoned. Knowing he won't be able to get in at close range and uncertain whether he would be capable of defeating Mayuri's Bankai even if he could, Stark will release Los Lobos and Mayuri will get smoked. Stark will unleash an unholy hell of Ceros from a distance and blast Konjiki Ashisogi Jizou out of existence. After that, Mayuri is easily dealt with: he can either overwhelm him with Ceros (which I don't think Mayuri is good enough to keep dodging; if Kyouraku could barely do it, he won't) or encircle him with his wolves. Mayuri will be either blown apart or he'll flee using his liquification ability.
It doesn't matter how: Stark beats Mayuri in a straight fight any day.
As for Matsumoto and Ishida, I'm unsure. I think Ishida may have the skill to beat Matsumoto, but it won't be an easy fight, that's for sure. Rangiku seems good enough to handle Ishida's arrows nor stupid enough to get easily lured into a trap that could cause massive damage (e.g. Sprenger). Furthermore, against Haineko's ash cloud, I think Ishida's arrows will be useless. Add to that some good sword- and Kidou-skills on Rangiku's part and she'll probably be the victor, though with difficulty.
Doesn't matter, though, because even if Matsumoto goes down early, Stark is more than enough to handle Mayuri and Ishida at the same time.
And with that, Team Big Gunz wins the Loser Bracket.
^ That brings an interesting point to mind: If Starrk is in character, he'll push the fight for Mayuri to release his Bankai which would be unnecessary trouble for Starrk, and especially Matsumoto. Still even if it came down to Ishida/Mayuri vs. Starrk, I believe the results would be the same with the only possible difference being Starrk probably needing to release himself.