The Elephant can train Marines and you start out with at least some resources, so train all Marines you can possibly have right now. It might even be better to wait a minute and replenish your resources (and thus build more Marines) before you take on the nearby Brutes. When the Brutes are dead (easier said than done), a fewSpartans will join you. * Note that on Legendary, the Spartans automatically come after you hold out for a few minutes, and they act as the Golden faction of the UNSC (the ones that you don't get to use). After the battle, they will switch over to your command.