When u hit someone, it'll display text such as, hit, smash, guard, crit, red crit. The damage % is as follows
Hit: 80% of max damage dealt
Smash: 100% of max damage dealt
Guard: 50% of max damage dealt (hit shield)
Critical: 200% of max damage dealt (back shot)
Red Critical: 400% of max damage dealt (only with zoomed in precision snipe skill + back shot)
with regarding to armors such as phase shift or i - field, damage is about 50% and all damage text turns small.
Note: These damage don't take into account the defense of your enemy. Which implies further reduction especially against units with fortress. That's why beam weapons does pretty much no damage against Psyco Gundam that has i field and Fortress EX on.
Be awed by my prowess!