If you've got a weapon with an attack level of 300 it will do maximum damage once you start fighting level 20 enemies, this is assuming you're level 20 or higher. Think of your light level as an additional part of your overall damage equation. A Fatebringer with 300 attack deals 1505 damage to mobs 20-32, if you're one level under you'll deal (1505x.68=1023), if you're two levels under you'll deal (1505x.58=873), if you're three levels under you'll deal (1505x.48=722). Levels are always the most important, but having a weapon with maximum attack helps a lot. A level 30 using a 235 attack weapon is like a level 29 using a 300 attack weapon. (VS level 30s)