The Project X is the monster with the third most damage per housing space, behind the Eye-ra and Slimeattikus. Couple with the fact that its primary target are defensive towers, the Project X is a perfect monster to send in with the Crabatron, which have high health, to take down towers. Project X is also good for the final attack on a base, when all of the towers are taken down, but the other buildings are still left. This is because it has enough health to substain the damage from one Booby Trap, which do 800 damage to the monster that runs over it. Since the Project X won't be dead, a Zafreeti can heal it. But don't forget, that Project X's main purpose is attacking and dealing with the defense towers, so that strategy might not always work out.
Sabnox is a better alternative, but should be used if high on Magma.
PPX is a commonly used method to destroy bases. However, due to an update, this strategy was nerfed. It is very powerful, but very costly. PPX means Puttied Project X. The best way to use PPX is with the 10M Putty Rage (otherwise it isn't quite PPX). It can easily destroy high CF (concentrated firepower) bases and BDBs (Big Dumb Bunch) in one wave although it can't destroy the spread design types of bases. Also, you can add one or two Zafreeti in case the Putty Rage wears off and the Project X are dying. Upgrading Project X to a higher level will also help through increased damage. PPX is normally used with Drull as a Champion since he can be puttied too with the 10m Putty Rage, turning him into a killing machine that can quickly destroy other buildings. The Zafreetis would also be able to heal Drull after the 10m Putty Rage has worn off. Visit the PPX page for more info. The new PPX is PS (Puttied Sabnox) due to its even higher damage by 200 (at max level for both monsters) aginst Towers, its attack range, which can beat Cannon Towers, Laser Towers, etc, and a little higher Health. Use PPX when high on Goo and PS when high on Magma.