And thus begins the final battle. You've made it all the way to the mad scientist himself, but he's not going to give up the Master Emerald so easily. The screen fades to black and a new section begins, though your ring count and timer remain the same. There are no additional rings to collect here, so if you've lost all the ones you had, then.. well, tough. If you die at any point during this final boss, you'll restart here, with no rings, but your timer will also reset too. Since Act 2 and the previous boss probably took quite a while to complete, you may get a "time out" during this fight, when you reach your 10 minute time limit. So ideally, you'll want to have completed Act 2 as quickly as possible, while still holding rings and preferably a lightning shield in order to stand the best chance of survival here. A bit unfair I know, but then again, the deck is supposed to be stacked against you in any final boss. Sonic's chasing Dr. Robotnik down a long, external road until the Doc eventually clambers into a massive grey orb thing sitting on the path. Sonic stops outside, as the door to it closes. The floor to your left is beginning to give way, and there's nothing but instant death awaiting directly below, so run for your life, all the way to the right. As you run past that orb, you'll see that it's actually the back of the head of Eggman's brand new giant mech, similar to the one in Sonic 2's final boss, but a whole lot bigger and badder.
When you get out of danger, the floor will begin to shake as the mech makes its way towards you, coming from the left. You'll see that it's now behind the path though, facing us head on while slowly inching its way to the right. It has its hands resting up against the path, each with three fingers pointing upwards. This is the first of three phases of this boss, but it shouldn't be too difficult for you, as long as you dodge swiftly and attack efficiently. These six fingers require three hits each in order to move onto the next phase, but you can land several of these at once. Basically, Eggman's plan of attack here is for the hands to move across the ground, trying to follow you as you run along. When they stop and lift up a little, they'll immediately come down, hoping to catch you underneath, just like the Lava Reef Act 1 boss (which is actually a very similarly designed hand). This is where you can take them out, but jumping on each one for each hit will take quite a while, as they're only down for a second or two. The more effective strategy is to stand between the two hands, and use your spin dash to bounce from one finger to another across the ground, which is a lot quicker. Between each time the hands clamp down, the mech takes one step in your direction, and the nearest hand moves as close to you as it can get. There will always be at least a small gap between the two hands for you to stand in though, so get in there each time and then let loose with the spin dash, which should allow you to get three or four hits in each time, on different fingers.
With the easy part out of the way, things are about to get a great deal tougher. He may not have any fingers left, but that's not going to stop the mech from cooking up a hedgehog-flavoured meal out of you, if you give him even a fraction of the chance. He'll move downwards, causing the ground to rumble. Stay on the right, because he's about to reappear from the left, with the massive mech head poking up through the floor, in profile. The floor will begin to crumble as he walks forward and begins to chase you, but the screen will scroll at the same rate, so you won't be able to outrun him. You do need to keep on the move though, because if you go too far left at any time, you'll fall through and die. While chasing you, the big red nose of the mech will snort out a small blast of fire every 5 seconds or so, which comes down and moves right along the ground, so jump over it while on the move. Immediately after it passes, you ought to drop back a little way, and jump and hit the nose, which will pause that flame attack pattern for the moment, and lower the mech's body, while keeping the head in the same position. In the space between, you'll see the Master Emerald! Surrounding it with machinery, Robotnik is using it to power a massive laser beam cannon from inside the mech. It begins to charge up, but before you attempt to evade it, you have just enough time to land your own attack. The Master Emerald is actually the weak spot of this machine, and you need to jump back and hit it, and then bounce off and run to the far right side, as quickly as possible. If you're playing as Sonic and Tails, with Tails being controlled by the computer, I find that he tends to hit the Emerald for you, just after you bop the nose, which is rather handy and leaves you to focus on the following part.
Now pay close attention. The laser beam, though it lasts for less than a second, blasts right across the whole area in front of the mech, horizontally, and there is no safe place to stand from it. Therefore you need to jump over it, but you'll have to do so at exactly the right time just before it fires, so that it disappears by the time you begin to fall back down. This is not easy. What you need to do is use the sound effect of the laser beam charging up as your guide. Jump as high as you can, as soon as you hear it stop. You'll have just enough time to launch upwards before the beam fires, but you have to be very precise. This is particularly tricky, but if you've managed to hang onto that lightning shield I told you to get earlier (though I wouldn't blame you if you got hit by one of those darn spikes in the Act 2 boss), it's more use here than ever, because Sonic can use the double jump ability to sail over the laser with ease, and good timing isn't necessarily required.
Same goes for Tails, who of course can just fly right over it, but if you're playing as Sonic without that shield, you need to jump at exactly the right time. Also, DON'T press pause while the beam is charging, because this will stop the sound effect prematurely when you unpause, and you won't know when to jump. After a moment, the body part rises again, and the whole procedure repeats, starting with another flame blast. As per usual, you'll need to hit the Master Emerald, and then dodge that massive laser a total of eight times to defeat this phase. It's one of your biggest challenges, but I believe in you..!
When you land that last hit, the mech will burst into flames and start to slowly topple downwards. The screen stops scrolling, so bounce off over to the right. The mech goes off screen and Dr. Eggman rises up out of the wreckage in his Eggmobile, desperately clinging on to what little hope he has left, as he clutches the Master Emerald in the grabbing attachment underneath. The ground is rumbling and the Death Egg is falling apart, with flying debris all over the place! You have one more phase to complete, but it's straightforward. Eggman doesn't attack at all, he flees to the right, taking the huge stone with him. All you have to do is chase after him, and jump and hit him a further eight times. This time the screen follows you, but you still have to keep running, as the left side of the long platform is collapsing again, and the only danger here is falling through it. Every time you jump and hit Eggman, you'll fall back towards it, so keep that right directional button held down nice and firm. Eggman will go off screen a little way, so keep running to catch him up. On your last hit, the Eggmobile explodes, and the Master Emerald will finally be dropped on the platform. The left side stops collapsing immediately, but there will still be a large gap there, so make sure you don't fall in (because that would be REALLY annoying). Eggman darts off, a defeated man, as the whole area starts to burst and explode, and the screen turns to white. Congratulations!
If you didn't collect all the Chaos Emeralds, you can now sit back and watch the ending. If you did, you've got one more mission to complete, in order to finally reclaim the Master Emerald. That's right, it STILL isn't over. Either way, go back to the Stages and Stories page to read up on the first ever "extra boss", in the form of The Doomsday Zone!