Port Razer and Helm of the Host on the battlefield.
Helm of the Host attached to Port Razer.
An opponent is unable to block Port Razer.
Steps
At the beginning of combat, Helm of the Host triggers, creating a token copy of Port Razer with haste.
Declare the Port Razer token as an attacker.
When the Port Razer token deals combat damage, it triggers, causing you to untap all creatures you control and get another combat phase after this one.