I found Asura as my top choice, or let's say, i usually used. Together with equip acc after summon (self/opp) = x2.2
It will be unlucky for me if the Asura just nullify the opponent's summon. But it only happens sometimes. Mostly Asura forcibly activates the opponent's summon.
I also use Carbuncle (match yours to opp) & Magic Pot (copy opponent's). These two summons save my butt several times.
My favourite summons : Shiva, Behemoth, DeathGaze, Ifrit, Omega hmm yeah those I guess.
It says it gives you the brave of someone when htey hit you with an hp attack. Does this work still when you are in break? Do they still do the hp damage to you?