Punisher

''Note: in early 2021 the name of this unit was changed from "Chastener" to "Punisher". However, the higher-level versions of this unit (Master-Chastener, etc.) kept the old name, as did the description.''

Punishers are an extremely useful unit in the early game, as they are significantly better than base-level stormtroopers, and you get enough of them in rewards that you'll have a pretty steady supply, so as long as you don't go too crazy, you should be able to use them in missions that you're having difficulty with. Eventually, of course, they will be surpassed by basic stormtroopers, but that time won't happen until you've gotten a fair way through the game.