Here's a macro I've been using an it's very convenient to use:
/micon "Dragon Sight"
/merror off
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/ac "Dragon Sight" <x>
/tle
You can just copy paste this straight to your macro in game. Just change the x into any kind of number your designated Dragon Sight is in the party list. If you set your party list to DPS > Tank > Heal, the next Melee DPS after you will always be in number 2. The /tle makes it so that when you successfully cast Dragon Sight you will target your previous target back.