If you're not sure how the Guard skill works, I've tested it out in all possible situations. I've also provided the same situation without Guard for comparisons sake.
Guard will prevent the enemy's special cooldown from reducing from any attacks if you start the battle while above the HP threshold. At level 3, the threshold is 80% HP. Even if your HP drops below the threshold during the battle, Guard is still active.
If you enter the battle while below the HP threshold, Guard will not be active.
There are 3 skills that directly counter the effects of guard. Currently that is Ayra's Blade, Heavy Blade, and Steady Breath. Ayra's Blade and Heavy Blade essentially have the same effect so the results are the same for them and all future similar skills. Same can be said for future skills similar to Steady Breath.
Guard slows down cooldown charge while these other 3 skills speed it up. When both are active, they cancel each other out in the areas they effect. Heavy blade and Ayra's Blade only effect the enemy attack, so enemy attacks will reduce special cooldown by 1 while your own attack will not reduce their cooldown. Steady Breath is only active when attacked and affects both your attack and the enemy attack so all attacks will reduce cooldown by 1 when you initiate on a Steady Breath user.
Guard will not prevent a special from activating if it's already charged.
-------------------------------------------------------------------------------------------------------------
__________________________________________________________________
If you like what I do, consider sponsoring me: https://goo.gl/dNycrr
Interested in other games? Subscribe to my second channel: https://goo.gl/MtJ3E1
Wanna chat with me and others who like FEH, video games and anime? Join my discord server: https://goo.gl/eSHNUX
Follow me on twitter: https://goo.gl/Xcjf5i
Want to sell your games but don't want to use ebay or Gamestop?
Trade your games to http://www.trade4cash.com and use promo code "cag10" to get 5% more money for your games. Trade in values are competitive for recently released games.(U.S. Only sadly)