I think the biggest reason your DPS drops is because there are parts in the fight that the uptime suffers. from midgar jumping away and doing his annoying mechanics to the tanks not positioning the exaflare in a way that allows people to keep doing their melee. Sadly the entire fight if you look at it has a bigger dip than O9S and O11S cause of these mechanics. It isn't something you really can work with. The only thing you can do is try to save up your abilities just as he is about to jump so you can try to do a full amount of damage in a short time like it is an opener. In the end though if your killing him in the end you are fine DPS wise and should enjoy the instance. I personally find O10S one of the most annoying fights they added mostly because of the large amount of derp I witness in the Party Finder.