I'm no expert in roam phinn, but I don't think a frostburn is good on any support, save... roam petal perhaps. Frostburn will only benefit Phinn if he lands his abilities, and that slow plus a bit of CP is not really worth the cost. If you want to keep enemies close or help teammates retreat, a warhorn or shiversteel can do those things as well as add additional defense stats, which in the the end is what Phinn should build up on over crystal power. As a roam.