Power Levels

Power Levels are a statistic that tells the game how strong a given Droid is. This lets the game be able to determine what a Droid becomes when rerolled, with rerolls giving a different Droid of the same Power Level. However, it also determines what type of Droids can spawn from a Mystery Button, or from killing a Droid with the Random modifier, with this case spawning a Droid with a Power Level equal to the droid with the modifier.

Enemies
yes there's a difference between enemies and droids, and yes you could reroll them, btw all "enemies" listed should technically have a power level of 0 (even if there's no proof they've been rerolled), which nowadays would result in a useless droid, although pre 1.5 would default to a weak droid (enemies and bosses are technically different which is why they aren't included, but they would function the same if they were rerolled, bosses have never been possible to reroll) (although I've only gotten some of the info either through my time playing this game, or even by viewing old videos and footage, although some of not all are no longer rerollable and only could be in old versions, with the most prominent time of rerolling these droids being the introduction of angelic builder, as its paint ability worked on almost every droid/enemy)