Since Wormzer has fairly low health and damage at low levels, send in a few Ichis or Crabatrons to distract the towers and then launch Wormzer. Always upgrade this monster, as it is cheap even at level 6.
It is highly recommended to use a Splash Damage Wormzer with an Eye-ra on a Bunker, as it is a quite good combination. Wormzer will kill all weak monsters and if the monsters are strong, then it was worth it to let Eye-ra explode.
If you have the Splash Damage ability, then launch Wormzer near defense towers or other buildings, as this will allow significant initial damage that affects many buildings next to it, including blocks. Then send in other monsters to finish the rest up.
A Wormzer without Splash Damage isn't that recommended for use, but if your Wormzers are level 6, send them in a very large number so that they have more effect.
It is recommended that Wormzer goes in with either Putty Rage or Fomor to help keep them alive and to make them take down towers even faster. However if done with Fomor, you must note that Fomor is vulnerable as Wormzers are burrowing and basically "invisible" to towers, leaving Fomor the only target.
If you are rich in Magma, use Wormzer's better counterpart, King Wormzer.