Pygmy Lily is a good option if you need to fight pests with varied weakness. If you fight pests with only one kind of weakness or not weakness at all (like Nidhogg), forget about her. Even if you fight pests with a unique non-Slice weakness, she may not be a good choice. Her ability give weakness attribute to only 2 allies. If Kerria don't get the needed ability, it's wasted. It's a pure 50/50. If you need to rely on that small chance to win the fight, you may found a better ally. I guess.