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.
Defeating Pelias
When Pelias shoots his beam at you, block until it ends. Then, shoot him with a spear. Repeat this twice. When Pelias flies around and shoots purple flying balls at you, hold [Block]. Wait until the balls circle you, then press [Power Block]. When Pelias stands on the ground shooting purple balls, stand near him and keep tapping [Power Block]. You should not take any damage.
Unlimited Dialogue
After receiving Hermes's Sight on Saria, go to Callas (outside the cave. Talk to Callas, then return to Agrios (in the village). You'll be asked to match three choices in order to convince him to take down the body. Pick a different god's response after the first dialogue choice (e.g. choose Hermes and pick Ares's option "He belongs in Tartarus") and Agrios will refuse to take the body down -- and that's just what you want him to do. Continue to make the wrong choices and you'll build dialogue choices for all the gods. These options satisfy the gods' dialogue quotas. This is an easy way to get Apollo's 75 choice deed.