Easiest way of doing this is
act_0123456789?fields=ads{insights.time_range({"since":"2019-03-03","until":"2019-03-03"}).time_increment(1){impressions}}
Remove .time_increment(1) if you don't want day by day data
you can use {} for subfields like
act_0123456789?fields=campaigns{ads{name,insights,adcreatives{image_url}}}
you can use . and () for parameters like; always make sure you use you use fields only after parameters like this order .(){}
act_0123456789?fields=campaigns.limit(1).time_range({"since":"2019-03-03","until":"2019-03-03"}).time_increment(1).breakdowns(country){ads{name,insights.time_range({"since":"2019-03-03","until":"2019-03-03"}).time_increment(1).breakdowns(country),adcreatives{image_url}}}