After the Phase Pods touches down at 5 minutes, the first pod opened will spawn the Raider. If no pod is opened until 20 minutes, the Leviathan will spawn in the bottom pod while the Raider will spawn top.
Defeating the Raider is usually a team effort in early-mid game. Unlike most creatures, the Raider will not regenerate health when out of combat, allowing you to kill it in multiple trips, although this risks the kill being stolen by the enemy team. Champions are not advised to try soloing the Raider until they are at least level 12, although this depends on the amount of gold acquired.