Spirit Punch/Seikentsuki does 2x the damage of a regular physical, but carries an element (let's call it Grapple for the sake of this post). Unlike most spells and skills, Seikentsuki's *accuracy* is impacted by enemy resistance instead of damage. That's why you see it missing sometimes, it has nothing to do with your speed stat. Jigospark is the other move I can remember off the top of my head that has its accuracy impacted by resists instead of damage, but there might be a couple more.
Other abilities that share this element include the Battlemaster's Spinning Sword/Samidareken and Rock Slide/Kansekiotoshi. A lot of enemies resist "Grapple" element especially later on, mainly because you would have no reason to use anything besides Seikentsuki or Samidareken otherwise.
My personal recommendations for attack skills are the Fighter's Spin Kick/Mawashigeri, which hits a group, and Swordline/Bakuretsuken, which hits 4 times at 0.5x power (so basically identical to Rapid Fire from FF5). These are non-elemental moves, so they will never be resisted, and they have no drawbacks (well, aside from ignoring most/all weapon side effects). It's hard to outdamage them without either getting relying on elemental moves/weapons that will be resisted pretty frequently by enemies, or incurring the damage penalty from Berserk/Sutemi.