When Crono is just about free after making his escape from the prison in Guardia Castle, a document is found on the desk of the Prison Supervisor. If Crono reads it, it would explain how to defeat the Dragon Tank, which he will end up fighting very soon.
The following is what the document says:
To the Prison Supervisor
"Dragon Tank Owner's Manual"
The Tank Head repairs body damage. It contains a shield that prevents damage by Lightning and Fire. Unless the Head can be defeated, the Tank is unbeatable.