Speak to Strahad Farsan in Ratchet.
I have taught you skills in the past, but the time has come for another to teach you. He is in Ratchet, and his name is Strahad. He is human, but do not let that fool you into thinking he is weak. He is powerful even though he may be small. He can teach you about more powerful summonings--I feel you are ready. Go to him now. He is in Ratchet... above the port town next to his tower.