The Melting Pot is a simple arena level. Before you head down, there is Something Nasty in the northeast corner of the barge level. You will have to break through the thorny Stabweed plants to reach it, though they hurt the Kid if he comes into contact with them. To avoid this, fire at them with a ranged weapon.
When you are ready, head down the stairs. You can collect the Trip Mine, and place them in the arena to start an initial defense. Feel free to destroy some of the Stabweed plants in the center of the arena in the process, as their thorny presence may make dodging enemies in the coming battle more difficult. Throw the switch in the southeast corner to start the battle.
The combat is rather hectic, with enemies either dropping from crates from above, or Squirts appearing out of the ground. There is a mix of enemies and friendlies, just to make the battle more chaotic. It will take about 3 minutes after throwing the switch before you can reach the Core locked under the cage. Until then, hold out against the attackers, and watch for any special pickups (such as the Ancient Spices memento.) Though this may be overwhelming on an initial run, keep in mind that this is an ideal stage to attempt a high score in Score Attack Mode due to the continuous waves of many enemies. Once you have everything, head to the barge at the bottom-right corner of the arena.