If you've already spoken to Nida and Dr. Kadowaki just keep resting in Squall's dorm. The King will appear eventually.

The reason SumMag+x% doesn't work for Diablos is that his attack reduces the enemy's HP by [Diablos' Level]% of their max HP. At Lv20 he will reduce their HP by 20%, at Lv100 he will reduce their HP by 100%. The damage is capped at 9,999.

SumMag+x% should not be used for Cactuar, Carbuncle, Cerberus or Diablos.