It cannot attack from a distance, nor can it jump very high. Use a ranged weapon from an out-of-reach position.
A Harpoon, Flail or Yoyo can be effective, as Nymphs can be stun-locked. Even with its high health, the Harpoon can kill a Nymph in moments with no risk to the wielder, due to its extremely high rate of fire, so long as it is used at short range.
Any high rate of fire weapon will be very effective, as they cannot approach you enough for their attacks to hit.
A Hunter Potion, Lifeform Analyzer or both can be useful for finding Nymphs.
Using a farm in the Underground Snow is a good way to farm Nymphs. A summoner is recommended.
In Expert Mode, the Lost Girl form has twice the health of a normal friendly NPC. This makes it suspicious even on a first time encounter.