Question

Rather the exercise was tiring is it grammatically correct ?

Answer 1

If you are trying to say that the exercise was tiring it should be

The exercise was rather tiring.

Otherwise to make proper meaning you can add something in front of the sentence.

In this case rather becomes a conjunction and not an adjective.

Answer 2

Rather the exercise was tiring. - "Rather" should not be there as the first because it is a onnector.