The wave itself is shaped as a UFO which consists of enemies, mainly UFO’s. It moves the same way as the Alien Mothership.
A part of the UFO consists of only one enemy type, which as follows:
Saucer - UFO chickens/UFO chicks
LED - Fester
Head - Enemies other than UFO’s (except gatling/laser)
Turret - Gatling/laser chick
Here’s a sample image:
Its harder variant has turrets attached to it: