Buy only legendary and 6 star runes with good stats and substats, check out next paragraph for stat review.
Look for speed and %primary stats as substats and for even runes look at the following main stats suggestions:
|4||%HP||Optimal for speedy support|
|4||%Crit dmg||Good for speed based damage dealers|
|6||%HP||Optimal for speedy support|
|6||%ACC||Good for speedy support|
Swift runes are really useful on monsters like: support , speed based skills monsters
Here is also a couple of monster with the appropriate link to the detailed stats and review