This is a shockingly easy achievement to get, but it requires you to realize that there's something of a trick to this fight.
Her leap attack - where she jumps up high in the air and bobs around, before zooming down to smash into you - is the key to victory. Stand still and let her bob around while shooting at her with your pistol or SMG; when she does the dive attack for you, if you shoot her mid-dive, you will counter her attack, which will stun her for long enough for you to get off 2-3 crack shots on her (your special Y attack while she's stunned).
Use your Trigger Happy ability when it becomes available, so long as she is close to you on the ground - it will be wasted if she's behind an obstacle or too far away, but it can do quite a lot of damage.
When she's attacking you on the ground, roll away, dodging as appropriate - when she jumps up to smash you, you want to roll backwards, while when she does a thrusting attack, you want to dodge to the side.
Once she's below half health, she'll turn red and start a different sort of leap attack that looks different from her normal one, playing a cutscene when it starts. This one is not counterable; you'll just want to roll around and dodge a bunch to escape it. Otherwise, however, she will mostly just use the same attack pattern.
I managed to kill her in under three minutes while using only a single healing item, so she's not terribly hard if you play this fight correctly.