Summoner primarily should be based on channeling Aether to Summon Primals as Pets and gain access to their elements.
Summoning
Ifrit- has hard mode abilities and gives the Summoner Fire based Spells
Titan- has hard mode abilities gives the Summoner Earth based Spells
Garuda- has hard mode abilities gives the Summoner Wind based Spells
Leviathan- has hard mode abilities gives the Summoner Water based Spells
Ramuh - has hard mode abilities gives the Summoner Lightning based Spells
Shiva- has hard mode abilities gives the Summoner Ice based Spells