When this level starts Cartman tells us that You can tab on an enemy unit to see what you're up against, and a finger indicates that you can tab on the shown opponent.
There was however no reaction to my tabs, no matter where I tabbed the screen.

The reason I write the last line in past sense is that as I am writing this the phone powered down, and when I reactivated it it was possible to tab the enemy.

The reason I post is that the same solution might work for others who feel stuck at that level.