On halo 2, On the great jounery where you meet up with the hunters and the ellite and go on to the bridge after the fight with brutes. i threw a plasma gerande at at drone and he flew back and killed the other drones \
Id say do encantment at mage training arena, do the best spell you can or the best one you can that uses one rune eg/ diamond enchant. Then use the points you earn to buy more cosmics.