Poison Lily, Nar Lily
Mysterious living flowers that can toss a blob of poison, paralyze non-Androids, and stab you if you're in range.
Hunters: These will irritate HUmars and HUnewearls the most. Daggers are not good against Lilies, so you either use a Saber or a Sword or Partisan. Why a Sword/Partisan? Apparently, Lilies have a secondary target that can be hit if you wield something that hits multiple enemies, but you'll only hit it on your first hit of a combo. The "attitude" of Lilies vary, so you might get lucky not getting paralysis. If you do, let the Lily shoot a poison blob at you so you can cure yourself.
Rangers: Your ranged weapons save you here, especially with Rifles. You could use a Shot, but as stated earlier with the Hunter strategy, you'll only hit it once. Try Mechguns in the later difficulties.
Forces: In Normal Mode, Lilies are more vulnerable to Foie, since Barta isn't that potent yet. But in Hard and Very Hard, you'll need to use Barta. Barta's range should keep you out of their paralysis range. For Nar Lilies, it's the same, though Zonde might work as well. As a side note, both the Poison and Nar Lilies are also vulnerable to Grants.