I'm not sure that I understand your question. 잘 하자 Is "let's do (it, this, that: depending on the context) well". 이번 is "this time". 에는 is a contraction from the markers for time 에 (at) and subject (는), which gives more focus on it.
In my understanding, this is a translation for " let's do well this time" but where the part "this" has gone? I mean we used to use " 이 것을" for something like " this (thing). So why its not included? Please explain!!