If you don't buy any green or better quality profession assets from the auction house or random packs, levelling up the people to actually run your profession missions is pretty time consuming. My characters generally have six of the nine available profession slots unlocked, so if I want to run six high-level leadership missions, I need six rank three minions. Since each of them requires four guards to be trained, and each guard requires four mercenaries, that adds up to a whopping 96 mercenaries that need to be recruited.
With my control wizard, I decided to plan ahead for once and initially dedicated all her time and resources to recruiting. It's not like the low level missions give great rewards anyway. By the time I'd recruited all 96 of my mercenaries and converted them into 24 guards, she was up to leadership level ten from doing that alone. Sadly that's not high enough yet to actually train rank threes, but at least there are already some decent missions at level ten that give astral diamonds and which I can keep re-running to level up further.