In this level, alien is coming to get the cows! Maxwell needs to save at least one cow!
Equip Maxwell with a weapon, wait patiently as the UFO arrives and take down the alien. Alternatively, you can drop some instant kill items such as “Magic” or “Red Magic” on the UFO and the aliens.

Round 1: Rocket launcher
Round 2: Laser gun
Round 3: Barrel gun

Scribblenauts Remix 11-2 Cheats Hints gallery:
11-2-1-a: Save at least one cow!
11-2-1-b: Destroy the UFO before the alien kills all four cows.
11-2-1-c: Use a missile launcher.