From what I learned as a summoner in fates are two things.
- Ruin II and Energy drain are excellent skills to grab mobs fast and deal damage.
- Summoners have one set of skills that none of the other classes do not have. A summon egi. Take advantage of it.
My carbuncle/ifrit/titan do fairly well in grabbing mobs first and doing damage. For all you summoners out there,
LEARN how to use your egi summons. There is a reason why there's a steady, guard, sic, heel and so forth. It's
your "2nd half" of your job.
I do believe titan and ifrit have their own little Aoe skills you can use too.

To be honest, you dont even have to do a large amount of contribution to get
the average rewards from fates.