Make it sound believable, read some of the statements and manifestos from aloha snackbar groups that they release (usually after attacks), and derive the phrasing of the threat from that. Otherwise Tor and guerrilla mail is probably enough to ensure anonymity, just be aware that guerrilla mail will likely be compromised by the authorities after the threat has been made, the email server that the LA bomb threat was routed through was subpeanoed by the German government and it can't be used anymore.