Hi,
Is saying: They beat heck out of each other - rude? Not very. The mild expresion 'heck'usually replaces the word 'hell'. But neither is the sort of thing I would say to the Queen of England, or even in a job interview.
Are there any other expressions close to this one? Yes, you can use quite a lot of quite rude words instead of heck/hell.
Best wishes, Clive