To throw a fit is not particularly offensive so far as the choice of words is concerned. It is either not a dysphemism, or, at worst, is only a very mild one. On the other hand, the judgement that this phrase expresses is often offensive to the person whose behaviour is characterised by it. The offense cannot be reduced by expressing the judgement in different words, because it is the judgement itself that is offensive to that person.
Somebody who throws a fit typically does not think of that as throwing a fit. The person may think that he is making a well justified objection against something, that he is legitimately protesting, or that he is manifesting anger for which he has good reasons. Characterising his conduct as throwing a fit expresses a judgement that his objection or protest is not justified, that he does not have good reasons for his anger, that he is being irrational. Moreover, the phrase expresses that judgement without offering a refutation of what he regards as the justification for his conduct; it summarily dismisses his reasons, without engaging them. Such dismissal will typically be offensive to the person, regardless of what words are used to express it.