Attack
Fenny's standard attack is a regular shotgun attack.
Being a shotgun, it has a short range but can break parts easily.
Standard Skill
Lionheart's Standard Skill causes her to charge forward, shooting nearby enemies when she arrives at her target.
With her Neuronic effect, this also grants Fenny a shield for 5 seconds if she has a Kinetic shotgun, which is a nice survivability bonus for her.
By-default using her standard skill costs 1 shot, which is quickly and automatically reloaded after the skill completes.
Getting her to Manifestation 2 removes this 1-bullet requirement.
This skill deals 33% of Fenny's ATK, 8 times.
Since shotguns fire around 8 pellets and hare around 30-35% Compatibility, this means her skill deals very roughly about the same damage as one shot of her shotgun.
... but this skill also grants shield, gives her mobility, and with M3 increases the damage of her next shot by 50%, so this skill is better than just shooting normally.
Basically just spam her standard skill as much as you can.
Ultimate Skill
Lionheart's Ultimate Skill causes her to fire in a line in front of her, dealing damage 7 times.
Hitting a targeted repeatedly causes subsequent hits to deal 50% less damage each time, meaning there's not that much point trying to hit a target lots of times - even 2-3x is very good and about as much as you'd want/need.
With her Neuronic buff, this skill does heal for 5% of the damage dealt.
This doesn't translate to a lot though, and if you want healing you're better off using a dedicated healer support operative like Eatchel or Quiet Quitter.
It is something though, and combined with the shield from her standard skill it can make Lionheart one of the more bulky 4* operatives.
Generally speaking, I'd generally recommends that you avoid using Lionheart's ultimate skill, and instead use your squads U-Energy for the ult of another operative in your squad.
Support Skill
Lionheart's Support Skill deals Kinetic damage in front of her, and also recovers some bullets.
With her Neuronic buff, each target hit reduces the cooldown by 2 seconds, up to 10s max reduction.
With her Manifestation 5, Lionheart's Support Skill also gives a 15% ATK buff for 5s.