Diplo: I walk up to the guards and use diplomancy
DM: Ok, roll a Insight check, normal DC
Diplo: 33! yeah!
DM: You realize these hobgoblins are professional Soldiers protecting thier camp. Letting you and your armed cadre pass would result in thier execution...
Diplo: Oh.. um.. Hey guard, I am a messenger with direct orders to speak to General Yahoo. Send those three lackeys to summon him quickly! {aside, hey guys, get ready to attack as soon as these minions are out of sight}
DM: That sounds like a solid plan, roll a bluff check to pretend to be the messenger, then a Hard Diplomancy check to get him to send extra bodies on a simple mission.