Ummmm... first of all you need to be a guild master. Then... your guild's level must be min. lv3. To buy a Mercenary scroll you need 5000GP and 500k gold. You can use them just in JOB (Thief,Hunter,Trade). They will kill just the players and not any NPC(Thief npc). Fire/Light/Cold mercenary use nukes, depends on your level. If you are lv120 they will use all nukes. Heuksal/Pacheon/Bicheon mercenary will use just some skills, not all. Pacheon will use Strong Bow series, I don't know the others cause I never used them. When you have lv4+ you can upgrade your mercenaryes with 2 abilities . Attack rating, Max HP increase, Max dmg increase or such things.
Hope this will help you. :p