Ukraine will need to push through the first and second lines of defense, where the Russians are extremely dug in. Once they breech those fortifications it should become like driving a scalpel through Russian defenses.

If they manage to go through the first lines of defense, they'll operationally encircle a lot of Russian positions on the west, either prompting the Russian forces to retreat or get trapped in the pocket.

TL;DR this is the hard part