So was running through some field maps on the way to a heroic mission the other day, and found a loot cave.. needless to say, I had some fun, and the level 34 in the video was jaw dropped and send me a friend request immediately after this. xD
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Sadly this only happens when the enemies move against each other. And you could've used Encore way better than you were. If you do that again, just continue using Showstopper. The enemies will spawn fast enough before it wears out. You can practically use it for the entire event.