Overview
M1919A4's Mod revolves around her Skill 2 and her GZ SPEQ 3-set. Her Skill 2 changes her targeting to be reliable, grants her a chance to knockback and stun enemies, as well as dealing 50% increased Damage to Giant units (
enemy unit tags can be checked here). Her Skill 1 also got a minor upgrade, but it's pretty minor in comparison. The 3-set bonus allows her to remove enemy HP Shields from Giants once per Skill activation, as well as lowering enemy Damage by 20% during her Skill (as her Skill has a very low uptime this doesn't do too much).
Removing enemy HP Shields is quite good, considering how plentiful they are. And most enemies with massive HP Shields are also Giants.
Her knockbacks aren't especially reliable, and there's the additional problem of enemies in attack animations not being able to be knocked back making it even less likely.