Despite what people will say about GLA losing aggro when leveling, it really shouldn't happen unless you're messing up. Flash generates 600 potency's worth of enmity which is more than any DPS can do in a GCD before you get Halone or Shield Oath. At worst you could spam Flash on any target. After 26, the entire Halone combo generates over 2000 enmity, meaning that you shouldn't have any trouble at all from 26-40. If you find that you can't hold aggro then either you're not using your enmity abilities correctly or you need to upgrade your gear, especially your weapon. PLD's damage pre-40 isn't low enough that the enmity modifiers are useless.