Gallaxhar's Clone Army are the cloned soldier versions of the alien warlord, Gallaxhar and the secondary villains in the animated film, "Monsters vs. Aliens". While having a minor role to appear in the movie, the Army makes at least minor fights in the villains wars.
Disney Vs Non Disney Villains War
Prior to the events of the first war, Gallaxhar and Emperor Zurg were major conflicts. To end the rivarly, Zurg launches a full invasion at Gallaxhar's ship, while the Gallaxhar himself produces thousands clones of himself, ready for the battle against Zurg. While the Clone Army takes on Zurg's Advanced Robots, Zurg himself confronts Gallaxhar personally. After a brief fight, Zurg sabotages the ship of Gallaxhar, killing all of the living beings on the ship, with the exception of Zurg, who trasported to the Animated Universe.
Non Disney Villains Tournament
Apparently, the Clone Army is not seen fighting, during the battle against Vector and Tighten. They are only featured, when Vector unleashes his dangerous ray weapon upon Gallaxhar's ship, wiping out the entire fleet of Gallaxhar.