You'll need to pull the caster mob from just north of Gaelas and let him attack you (just leave the mob on ~20% HP then run to Gaelas), he'll take the Frost Cage off you. Then you only need to keep your interrupt for Firestrike and you'll be fine.
If you have major FR gear you can simply tank the Firestrike, but if you're playing completely solo you're better off just skipping Gaelas completely for now. The quest won't give you EXP anymore anyway.