Okay, here's my story.
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I went in completely underprepared. I accidentally only took eight Revivier Seeds with me. However, I decide to keep on truckin' and reset if things didn't go my way.
I only lost THREE Reviver Seeds in total. One time I was KOd by a random Charmeleon's Fire Fang, and another time we got separated by a Warp Panel and my Vulpix went down to a trio of attackers.
When we finally got to Darkrai, I was pretty tense. I only had six Reviver Seeds. However, I did what you suggested, tossing a Stun Seed at Darkrai, immoblizing him for the rest of the match. Then we went to work on his henchmen. Imprison was invaluable, immobilizing Arbok, Magmortar and Aggron. I swiftly took out Rhyperior and Magcargo with Brick Break. Behind me , Vulpix Fire Blasted Aggron. This left an immoblized Magmortar, a Mismagius (who atm was slaughtering Cresselia), and Arbok. I 2HKOd him with Wide Slash. Cress fainted and was revived. Vulpix KOd Mismagius with Extrasensory. I used Rock Slide for the KO on Magmortar.
It was just us and Darkrai. I wasn't looking forward to the inevitably ugly fight to come, when I remembered. I had a Vanish Seed from my previous venture! I ate it, then defeated Darkrai in four hits while he just looked at me (or where I should have been). It was an easy victory, much easier than in Darkness. It was all thanks to your strategy help. Thanks a mil!