Its a pretty movement heavy fight so you really shouldnt be worried about low dps in a fight like that. Id say as long as you are doing dmg and arent taking any or too many hits then you're being helpful to the group.
As for cooldowns to be used, id say use Cds when you need to burn the sisters and when ever you need to push her into the next phase.
Garuda EX is a really straight forward fight. Its titan thats a real test.