If the OP has access to a heat gun, that might be a better option than potentially voiding coolant, or scalding themselves.
I'd recommend keeping a flame away from the area, so that the plastic in either the cap or the rad doesn't melt / fuse together.
Some medium hammer taps on the top / sides of the cap may loosen things a bit.
And of course, push down a bit while turning. Might need to push down a bit firmly, vs the usual amount needed.
If all of that fails, and the channel locks crack the neck of the rad, some jb weld for plastics may repair it, at least in the short term. I used some on my headlamp housing the other day and it's still holding (although my headlamp is waaay out of alignment at the moment, and I need to see if I can re-aim it - if not, then the time spent trying to fix it will be for nothing)
Finally, it might be a good idea to have new cap on hand, especially if there's no backup transportation.
Good Luck!