Both of these achievements require that you have Yoshimo in your party when you are ready to advance to Chapter 4.
Friends Like These will unlock when you kill him while in Spellhold. The other, Peace At Last, will require you to loot Yoshimo for Ysohimo's heart after the fight with him. Once you reach Chapter 6, bring the item to the Temple of Ilmater in either the Slums District or Waukeen's Promenade. Leaving Yoshimo behind at the end of Chapter 3 will void acquiring these achievements.