Due to Omega XII's attack patterns (ie only using attack) it's very easy to beat him by abusing the famous Reverse/Lure stratagy. At high enough levels you can basically set your gambits and let your characters do the work for you, but even at lower levels (60-70) the boss fight require minimal input from the player.

Yiazmat is similar. After setting semi-decent gambits the only time I had to touch the controller during the first half of the battle was when my party leader was killed and the game asks you to select a new one. After half way I actually had to input commands every so often due to deathstrike spammage but even then things were relatively straightforward. It not until his last 5 million HP or so when he actually becomes challenging and if it wasn't for the fact that you can leave the arena without consequence he would actually be rather hard. As it is though, the fact that you can leave to re-stock Chronos Tears and recover your MP means that the last leg of the battle can easily be undertaken in several smaller chunks which drastically reduces the difficulty.