This is what boring does.
Yesterday I was about to loose a fight with the Grotto Greygnarl (Priest&Mage killed, 1 Yggdrasil Leaf left), and I discovered some kind of strategy...
First, Greygnarl gets enraged by anyone who uses Zing or Kazing. My Minstrel was with full HP (730, mastered with Skills), so I casted Zing on my Priest. It worked, and like I said, Greygnarl getted enraged by my Minstrel. That let me to use the Yggdrasil leaf and Omniheal with my Priest (look, the Warrior was about 80/710HP and the Priest with 113/423 and she had lots of MP, and Multiheal wasn't enough).
Magic Bursts: Before casting a Magic Burst, Greygnarl uses an ability that increases his attack, defence and agility a little. Exactly after that, he uses the Magic Burst with Disruptive Wave and MP recovery. He did the same every time I fought him, so I getted some Yggdrasil Dews and Leafs with me. It is kind of boring if you see.
Best strategy to defeat Greygnarl:
Funereal Fource casted by anyone who owns the Armamentalist's Album. It will give you about 1.5% more strenght when you attack. Gritty Ditty/Oomph casted to your Damage Dealers, or DDs. Falcon Blades/Uber Falcon Blades with Falcon Slash for 4 hits per turn (Fan Dango, Multithrust, Multifists and Rain of Pain also work well). Right as Rain casted by your healer, and Multiheal every turn.
Recomended Stats & Equipment
Everyone should have about 600 or more HP (Warriors/Gladiators with maxed HP get a +300HP bonus, so they will be about 850HP. It is recommended that your healer has got maxed HP and Resilience with Virtue).
Falcon Blades/UFB to your Damage Dealers, Silver Shields (Saintess Shields, Goddess Shields, the Metal Slime Shields Series and Brain Drainer family if you are a Grotto Veteran), good armours (I recently gaved my Priest the Wear-with-all-award, because I was too lazy for making another Divine Bustier. She wears a Pallium Regale that Goresby-Purrvis gaved me in a DLC quest). Shoes with high evasion rate for everyone (Metal Slime Sollerets Series, Sensible Sandals family and Pixie/Tricksie boots). Meteorite Bracer for everyone, unless you've got Goddess Rings or Life Rings/Bracers.
That was all, folks! See you later with more strategies, walkthroughs, and an upcoming Grotto Recopilation... Hope I help you.