I think you are missing because some MVP's def is higher than your atk so that end up making your dmg less than 1. Since most of the MVPs have high def, one way to solve this problem is to equip a Memory of Thanatos card on your weapon, which make your physical dmg higher when your target's def is higher. If you are talking about missing physical attacks when you dropped a MVP's health down to 20~30% or below, then that's the case when you need more hit/phreeoni card since most of the MVPs will use Increase AGI or AGI UP when their health are below 20~30%. Their flee will be doubled or sometimes even tripled after using that skills.