Advance to the River or other defensible terrain using heavy forces. Dig in and hold with lighter forces allowing the (now more experienced) heavier forces to be sent elsewhere. RUS responds to the incursion and hits the dug-in troops in defensible terrain who hold the line as best they can, while UKR diverts the heavier forces to another site where they wash/rinse/repeat. Taking the initiative, putting RUS in the place of being forced to attack in a place of UKR's choosing.
Then when UKR determines where RUS has pulled troops from to do the above attack, they send those heavy/mobile forces into an attack in that area and do another advance.
Seems like this might be the end of the Russian summer offensive, and we have a fall Ukrainian offensive happening.