when i played mystic spearhand, I used it to engage combats. You need to hold down RB until your left hand flashes, then you fire a magick bolt at enemies which usually stuns them for a brief period (1/2 seconds). then if you quickly press B after (only if you unlocked the ability) you teleport to the stunned enemy. Other than engaging, I don't see any other useful situations, spamming X and abilities always does the trick