Since Black Holes are one of the least appearing objects in the game, why not have another wave with Black Holes?
Trapped in Rivers is a challenging wave. We have 12 Black Holes, the left river goes up and the right river goes down. You have to use the anomalies to destroy the strongholds on the sides. There are stronger enemies there like gatlings (I used flamethrowers)
But be careful! LV4 Barriers make it difficult to win the wave, the ones in the corners are there so that the player does not have too easy!
Wave appears at 90% of difficulty.