Defeat Achilles with the pre-selected spear by dodging to his side or back when he charges Jason and thrusting out once with the Lethal blow. If the hit spark is gold or yellow, the hit will be a dud, but occasionally you will score a bloody injury.
A second method is to block Achilles as he's incoming and then counter-attack twice with Wounding (weaker) blows. This process is just as long as the first; the champion of Hellas doesn't drop dead easily.
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.
Unlockable: Children Are Annoying Achievement
On the island of Mycenae, the party will meet an orphan named Bolo. Endure his singing (by not selecting the choice ' Enough!') enough times to unlock this achievement.
All the other achievements in the game are associated with the various tasks detailed in the constellations menu (select a masked task to reveal the details on how to achieve them).