Sleep Sprout has Auto Regen and has a high chance of an ambushing the party, beginning with Goodnight, inflicting magic-based damage and Sleep (by 99 turns), Poison, Berserk, Power Break and Armor Break. Goodnight will also be used as a counter to all physical attacks.
Being rooted to the ground, Sleep Sprouts have zero Evasion. Sleep Sprout's attacks consist of the third-level elemental spells, along with Flare and Ultima. It has no elemental affiliation and is weak against Fire.
It drops weapons with status strikes and armor with status proofs, the Sleep versions of these abilities being fixed. Its equipment drops provide a hidden 6% critical hit chance bonus instead of the usual 3% (this is relevant to weapons and armors, but only to attacks that deal physical damage).
For all except Ultima, using Reflect is a handy defense. As Sleep Sprout has less than 99,999 HP, a strong party member can kill it in one hit, making killing it the fastest way to farm Dark Matter. The best option is using Celestial Weapons at full power (if available).