Defeat Pelias by throwing Jason's spear towards him (at first) after he's exhausted his magic beam against Jason's shield (block the beam's whole duration). When Pelias changes his attack pattern and shoots magic missiles from the air, power block (guard and press the shield shove button) to reflect the missiles back at him. When Pelias changes his attack pattern a final time and shoots magic missiles from the ground, simply dodge his attacks and rush him with melee weapons. Destroy his shield with melee attacks and fertilize his the ground with his fetid corpse.