I'm a mid level player, using Prophet mainly. There is a technique for stopping Prophet rushes using 3 spartans you might like to know.
First up there should be a couple of flame turrets. This will force the Prophet to mount his attack on your base's blind spot. Don't panic, let him attack supply pads, until you have 3 spartans. Then send them out and have them concentrate fire on the Prophet (don't get too close). If he wants to beam them he'll have to get close, retreat them towards your flame turret so Prophet comes into range and gets extras damage.
If he starts sending in honour guards again retreat your spartans towards a turret so the guards will get flamed by it.
If he makes the mistake of sending in Wraiths or Locusts jack them.
The Prophet is strong, so a spartan can easily die during this. Just replace it.
Do this well and the Prophet will take a lot of damage, and have to recall.