Aika is a very basic mmorpg, you only need a little bit of communication to know when to stop and recover, most dungeons are pretty straight forward but some people has some problems when starting the upper level dungeons.
The ideal composition would be to have all 6 classes to gain the perfect party buff which will make your party 50% stronger and double your luck; now let's get on to the roles:
Basically what it means to be an initiator is to take the first damage of the mob and to pull when you feel capable of taking a big group, do not attempt to be the main tank or to pull aggro when you party can only handle 1 mob at the time.
The paladin role is to soak up all the incoming damage except for the aoe damage and some party de-buffs, must be exactly at the end of the range of the casters and dps and constantly switching aggro from mob to mob in a group sharing it whit the Warrior, as long as he can handle it too, remember that you can heal yourself but this must be avoidable at all cost because it interrupts the flow of the battle and has a high chance to take the aggro off you and onto the casters.
[DPS Burst]Rifle man.
Animation canceling on the rifleman is not very useful, tough it might be recomended for some skills, so you don't actually have to move around too much, take advantage of your Nuke strikes and don't feel bad if you think someone is doing more damage in the long run you should be the nº1 damage dealer at the end; you have also a secondary role, keep an eye on the back of the line and the surroundings, in case a patrol is heading your way, and to protect the cleric at all cost.
[DPS De-buffer]Dual Gunner.
Animation canceling is a must, don't spam all of your skills at the same time, allow to a similar de-buff to wear off because you are very mana reliable and healers mana is very precious to be wasting it on a mana regeneration spell, most of the time mobs will try to take aggro on you because of the way the game takes dps aggro over most damage dealt, so stay away try to control your movements, ALWAYS bring ammo.
[Back of the line]
Oh the magnificent lolipop, the main role of the cleric is to heal in this order Paladin>Warrior, and to keep an eye on the de-buffs and cool downs of team mates and mobs, if your party needs some mana try to time the spell with the warrior taunt or a paladin pull.
The warlock is the AOE grinder by excellence, you might not have enough power to kill a mob in a single hit but you can take a full patrol in 5 seconds, no other character can say the same, your job is to map out the dungeon before hand and lead the pulling from behind, you must know the damage output of your party since the very beginning and take care of the healer, if the cleric dies is your fault, don't waste time using de-buffs on single mobs unless it is a Boss or the aggro is in danger.
That's it folks! keep an eye on everyone as much as you can, ask for advice, always bring ammo.