Unlock "Children Are Annoying" Achievement
The party will meet an orphan named Bolo on the island of Mycenae. 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).
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.