Full analysis
Both her damage multipliers are after Armor, so she struggles a lot with Armor.
The damage multiplier applies to the first attack on marked targets. The 1.6x multiplier stacks multiplicatively with itself, so she ends up dealing 4.1x damage per attack from the third attack onwards.
While any Marked target exists, DesertEagle will lock onto one of them and keep attacking it until it is dead. Wiggling her will not change targets, and she will stay locked onto that target until it dies or the Mark expires (such as if enemies with more total HP and HP Shields come into range when the Skill is activated again).
Dealing damage to both HP and HP Shields is largely irrelevant, it is bypassing the HP Shields that matters.