The question is does the concept "tough" invokes the same meaning in an Esperantist's mind as in an English speaker's mind. When you say "life is tough "you mean it is hard. When you say "I'm tough" you mean that you are strong. To translate your sentence in Esperanto you need to use a word that contains those 2 meanings. Translation is only possible if there is such a word. I'm not sure whether words like "forta" and "malmola" contain those meanings. If yes then the literal translation would be "La vivo estas forta, sed mi estas pli forta"
Sure! In my understanding, Esperanto is the most flexible language that is. Werevrock is absolutely right, and the solution of a literal translation is smart. I think that la vorto "malmola" contains the meanings "hard" "difficult", and "tough". In that case, we could translate the phrase as: "Vivo estas malmola, sed mi estas pli malmola". Cxu?
"Tough" means two things in the sentence "life is tough, but I'm tougher". First it means "difficult", then it means "hard, durable".
This sort of saying is pithy in English because you can play with that double meaning and make this concise little sentence. But there's no word in Esperanto that has these two meanings... you can't translate this into a nice pithy little sentence in Esperanto.
To get the meaning across, you'd need to say something like, "life requires toughness, but I am more tough than necessary". La vivo postulas malmoleco, sed mi estas pli malmola ol estas necese.
*I believe we need necese to be in the adverb form because I don't think we can point to any noun in particular that it's modifying (and that's what Zamenhof did in these cases). But necesa would get the meaning across just fine.