Priesties(Priest) - They seem pretty self explanatory, the priest class pretty much heals, buffs and heals A LOT!
Wizzies(Wizards) - Your job is to hopefully kill or weaken the enemy while they are coming into the emp room. Guild Masters: Never let all of your wizards cast the same spell. A max should be 2 wizards for each spell. All wizards should be put together, and should never be apart(usually in a line for Crusaders, more on the below). The wiz line should never stay near the entrance, more around the middle of the room, where they can easily cast at the entrance.
Bards - You cast strings to help the wizzies, duh.
Dancers - You cast slow grace near the portals. So your wizzies can put a hurten on them enemies a lot more.
Dance & Bard - When both are together, both dancer and bard go near the portal. The dance casts slow grace, but with the bard next to her, it turns into classical pluck. Classical pluck makes your opponents and allies not have the ability to use there skills which is great for the opponents who send there casters (so beware for melee).
Sages - You pretty much cast Magnetic Earth on the wizzies to protect them.
General Melee: Knights, Crusaders and Blacksmiths - You guys pretty much stay around the entrance of the Emp room, the middle of the room & near the emp. You litterally kill the living hell out of anyone right next to you.
Crusaders: You are considered back up healer when necessary. An essential thing any guild needs is a Crusader. At least one needs to Sac the wizard line. So damage on them is put on your massive amount of hp.
Hunters - You pretty much are used for trapping up. Pretty much ankle snares and shockwaves on the walk up to the emperium. Ankles =slows down opponents Shockwave= So you can see those darn ninjas. For damaging, you stay slightly farther back from melee and just snipe anyone in range.
Ninjas - casting blaze shield near portal to stop ppl from advancing (no int necessary use stats for killing strike) and using killing strike to finish ppl.
Rogues & Assassin - can stealth and what not. You are the person in charge of stopping recall from other guild mates, use stone fling. Other Rogues stay near portal and close confine + stealth lord knights or other fast emp breaking ppl, if they can get you out of stealth then do close confine again but instead strip them. Assassins with edp or link kill as much as you can.
Monks - You are just like the melee but you hang low. When chaos, destruction, horror is right in your face, it's time for you to react. Find the number one badass (mainly the person hitting the emp the fastest) and star killing him. Tip: use that g-fist move. (Hide clip close to emp would be awsome!)
Other stuff - Always keeping suff and pnumea on the wizard line, because you just need to ALWAYS EVERY SINGLE TIME YOU DEFEND. Keep doing it, until it's not necessary or your guild master tells you to stop.
Soul Linkers - Your the person that just links every single character around you. Or for a better term, "you give buffs that make your teammates hella uber",
Most credit goes to
http://strategywiki.org/wiki/Ragnarok_Online/Walkthrough