Lava Reef is the fourth zone in Sonic & Knuckles, now expanded with new features for Sonic Mania. It takes place in a volcanic underground setting. For the achievement/trophy of this zone, you'll have to maneuver a giant walking mechanism through lava, destroying some Rexons (snake-like enemies that sit in the lava) in the process.
Act 1[edit]
Smash through some boulders and hit a switch in a nook on the rocky ceiling to open up the path forward. Destroy a Rexon to use its body as a platform atop the lava (a tactic that should come in handy throughout the zone), then hop onto the walker. To keep it moving, hop onto its other leg and it will swing forward, crashing through hazards in your path; repeat the process until you reach a dead end. Be mindful of the fireballs shooting out of the wall as you climb up the stairs. After crossing a bridge and some crumbling platforms, you should spot a rock that you can step on to reveal a spring, giving you access to a great amount of rings up above. Back below, you'll have to hit another switch to open the path forward. Once you come to a stop, you should spot a Fire Shield in the ceiling; in a zone such as this, the value that a Fire Shield can bring should be clear. Eventually, you should reach a lift that you can operate using a continuous Spin Dash. This will take you into the depths of a cave, as well as the first Star Post.
After escaping the cave, a massive drill will cause the ground to collapse. If playing Sonic, try using a double jump to charge toward the Item Box (Invincibility) on the other side of the drill (as Tails or Knuckles, simply fly or glide to it). This should soften the blow of the fall down into the lava. A switch to the right will allow you to enter a cave. The biggest hazards here are spiked balls and lifts that attempt to crush you. Upon exiting the cave, you should spot the second Star Post. Continue along the path to an encounter with the act's boss, the Drillerdroid. To defeat the Drillerdroid, lure it to the platforms that sit above lava. This will cause the lava to erupt right into the machine, loosening its defenses. Do this three times to expose the machine's sides, allowing you to deal damage to it with some simple Spin Jumps.
Act 2[edit]
You will now find yourself in the crystallized depths of the zone. Try to stay low to find some useful Item Boxes (including an Extra Life that will attempt to escape your reach on a conveyor belt), but regardless of the path you choose, you should eventually come upon a walker in the lava. Keep it moving so that it doesn't sink; when you hit an enemy, the achievement/trophy should pop. If you don't have a Fire Shield, you'll have to rely on the walker and some floating boulders to get across the lava, but otherwise, most of this act's gimmicks remain unchanged from the first. The boss fight is where things get tricky; Sonic and Tails fight the Heavy Rider while Knuckles fights the Heavy King. The Heavy Rider will be riding around the area on a Moto Bug, and must be hit with Spin Jumps mid-ride. The Heavy King on the other hand has much more variety in his attacks, stopping only to recharge. This is the best time to attack him. Hit both bosses eight times to defeat them.