In conversation, in this context I’d probably go with the phrasing:

J’imagine que tu n’en es pas à ton coup d’essai, mais dès que tu te retrouves en pleine nature, sois paré à/pour toute éventualité !

Though not a literal translation, it carries the connotation of “not green anymore”. As an aside: The pronoun “en” refers to whatever field you’re involved in or whatever activities you’re engaged in, so you don’t need to add a phrase like “dans ce domaine”.

You could say this in a number of ways, but my instinct would be to say:

Je suppose que tu n’es plus novice dans le domaine ; néanmoins, lorsque tu te retrouves en pleine nature, sois prêt(e) à toute éventualité !

Notice that I prefer “le domaine” as opposed to “ce domaine”. Whilst you can definitely say either one, using the determiner “le” in this instance seems more natural than the demonstrative pronoun “ce”.

In addition to the other (good) answers, another idiom may fit:

Je suppose que tu connais la musique, mais lorsque que tu te retrouveras en pleine nature, sois paré à toute éventualité !

Connaître la musique means that you expect the person to know all rules, habits, guidelines, etc, because they have done it several times.