Having trouble Getting and holding aggro after level 50

[Galgarion]

Have you completed the main story scenario? (By which I mean have you reached a point in the game where it wants to send you to Ishgard after retreating from Ul'dua?) If you haven't, stop running dungeons for a while and work on those story quests. You'll be rewarded with Item level 110 armor and weaponry that will make things a lot easier for you. There's a particularly nice ax called Conqueror that should solve your enmity problems in 50 content and below.