Buffer

Buffers are builds that stand in the back, making sure everyone in the party has the best buffs possible. There are many options for which buffs to bring, but remember that each party member can only receive a limited number of buffs. Every party member should inform the buffer how many buffs they can receive and what build they are so the buffer can plan accordingly which buffs to give to each party member.

Offensive buffs
Song of Fire is one of the strongest buffs in the game because it increases attack speed. Almost any build has use for it.

Song of Thunder (for casters) and Precision (for fighters) increase crit rate. It is best used on targets with high crit damage.

Enchanter active skills, such as Imbue Fire, add elemental damage to weapon for a certain number of hits. They are best used on proc builds.

Defensive buffs
War Cry increases maximum life. It can often more than double everyone's life. Anyone will be happy to get this buff.

Stoneskin is also a buff anyone has use for, since it increases physical resistance. All enemies deal physical damage and it constitutes about half of the damage they deal.

Song of Ice increases accuracy and defense. It is best used on targets that have defense multipliers, such as priests with Shield Mastery, warriors with Blacksmithing or Warlords with Battle Roar.

Battle Roar increases defense by a certain %. It is best used on targets with heavy armor. Combined with Song of Ice, it gives even more defense.

Epiphany increases mana regeneration. It is good for anyone that otherwise runs out of mana in a long fight.

Pact grants life leech. It can sometimes give your damage dealers just enough life leech to survive.

Vitalize, Prophecy, Insulation and Hallow are situational buffs against certain enemies.