Sonic 3 & Knuckles: The Challenges: Part 12: Knuckles' Final Battle (S-Rank)
78Final Battle has three separate challenges for itself, and each of these challenges focus on different characters facing several of the latest bosses in all of Sonic 3 & Knuckles. Just beat every boss your character must face within the time limit in order to complete the challenge!
The first Final Battle focuses on Knuckles.
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First, Knuckles has to go through Lava Reef's Act 2 boss, which he originally skipped entirely and was only accessible as Sonic (& Tails).
This entire sequence and its subsequent fight plays out exactly the same, though. Just grab the Fire Shield and don't get hit!
But after Eggman is dealt with, here, the lava immediately cools off, and Knuckles can move on.
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The second part of Knuckles' Final Battle takes place in Hidden Palace Zone. This time, rather than his own shortened version of the level, he finally gets to experience the larger version of the level that Sonic did, including Sonic's own fight against him.
But rather than just fighting another Knuckles, the Knuckles boss is much darker, faster, and stronger! He Spin Dashes sooner, glides faster, punches fireballs, and tosses Launch Base bombs!
He goes down the same way, though, and once Dark Knuckles is knocked out, he gets up, and runs away.
Then you, as normal Knuckles, just has to make your way to your own underground passages and reach the teleporter that takes you to Sky Sanctuary.
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The third and last part of this challenge has Knuckles back on track through his own version of Sky Sanctuary. This means finally fighting Mecha Sonic in this hack after he was gone for Tails' course through Sonic's Sky Sanctuary in the High Voltage challenge. The Egg Robo that chased Knuckles around even finally makes a reappearance... just to get wrecked by Mecha Sonic all over again.
This also means that you just have to finish this Final Battle challenge by beating Super Mecha Sonic!