arts...
for melia summon bolt, summon copy, summon fire, maybe summon wind, spear break, starlight kick and hypnosis (mostly for breaking visions), either mind blast or reflection (against visions)
for riki the 4 damage over time spells (poison, ice, fire and bleed) - I don't know how they are called in English. also his healing spell and... maybe hero time?
for 7th... I guess 2nd gear, and everything that looks like it could do some serious damage
if you don't know how to play Melia properly, I found this old guide on gaf. it's still pretty useful:
http://www.neogaf.com/forum/showpost.php?p=34985554&postcount=16970
(you can also use shulk instead of 7th if you don't feel confident about dealing with visions without him)