Io can stun Terminus by Luna through his siphon. Damage she deals from weapon is higher than from calamity blast and if she bought nimmble she is uncatchable without Terminus' cards on speed. Without his assistance you shouldn't be afraid of him.
Io's ultimate provides her invulnerability for a prolonged amount of time if you are trapped with a Terminus resurrecting using the ult to invuln through it, similar to Androxus' Reversal you can use your ult to wisk Term away before he can react.