Full analysis
Passive: At the start of battle, randomly select an enemy unit as “prey”. Contributing to killing the prey increases self Damage by 45% and increases Accuracy by 6 for the rest of battle.
Note that the Accuracy buff is a flat +6, not multiplicative. This does actually make it far better for Night, though obviously doesn’t make her much more usable.
Savage 99’s Damage buffs are both multiplicative with themselves and each other.
To be credited for a prey kill, Savage 99 must have dealt damage to them before they die, and they can’t be one-shot by her. (So if you somehow manage to stack her buffs up, and she starts one-tapping enemies she can no longer gain more buffs.)
Prey markers only apply to enemies in firing range, so if more enemies are further away you won’t get a marker stuck on one of those.
Her Prey markers are unrelated to the ones that are applied by Beowulf.