King Wormzers do very large amounts of damage when they surface, easily destroying black diamond blocks in groups of 5 or 6. It is recommended when planning your base that you have your yard spread out, so they can't destroy as much when surfacing.
King Wormzer is not the best when it comes to fighting a pack of Grokus, as they can easily die. It would recommended using him with Sabnox to distract the Grokus and then sending out King Wormzer.
In the Overworld, if you attack 12M Outposts send at least 10-20 King Wormzers paired with Level 3-6 Fomor. Then watch the outpost. It can destroy it as fast as 30 Seconds to 1 minute faster than D.A.V.E. It's better if your Fomor's level is Level 6 Bonus 3 because it gives 75% buff to King Wormzer. If you have too little magma, send only 3 King Wormzers.
King Wormzer is not only great for attacking. It is a good idea to have some in your main base. This is because he can cause to damage to all the monsters around it when he surfaces.
It is a bad idea to make this monster with low-leveled Magma Pumps. Also use this only when needed, as its expensive cost can bring your Magma down to a very low value.
This can also be used in the Overworld when bunkered, as a Level 4 Monster Bunker filled with 6 King Wormzers can deal a staggering 28800 for its Splash and along with an Eye-ra, it can add 24000 damage, damage able to kill a Fully Leveled and Bonused Drull at ease as well as with Gorgo due to King Wormzers health and raw damage.