The most annoying aspects of Evrae were it's large amounts of health and its rather irritating resistance to fire, water, thunder and ice magic. I remember finding it annoying initially, but not terribly difficult. I recently replayed FFX - picking up a save when I was using every character in every battle - and easily killed it when I knew what I was doing in the game. Slightly disappointing, but still a very fun boss battle.

Most difficult boss is most definitely that Spectral Keeper before Yunalesca. Very irritating. I remember finding the third Seymour battle very difficult initially too.