Zagnoid is just like a buffed Octo-ooze, with the life of an Ichi because it has more damage and more health. When using him against towers like the Blast Tower, fling 1 Zagnoid in near the tower, followed by a larger group, the Blast Tower will target the first zagnoid, while the others destroy the tower. They have a x2 damage bonus when attacking defensive towers.
When attacking a Quake Tower, fling a large group of Zagnoids as close as you can from the tower, so they can destroy it before it makes 2 earthquakes, which are enough to kill all your Zagnoids.
Zagnoids are better in large groups, and can be very effective when combined with Spurtz.