You need to kill specific units, only some of them count as part of the final wave, the rest are just chaff there to exhaust your resources.
The final wave thrashed me in my first attempt as Undivided, but I found that pushing everything except a handful of units left at the first capture point to hold off the trickle of reinforcements from the bottom of the map up to the end zone to fight the last wave worked. Course I had fliers to take the fight to the air, which Kislev doesn't have the option of.
I think you kind of have to spend some time building up points to call in reinforcements before triggering the final fight though, not long, just until the extra points for holding the most recent capture point stop coming in. Towers and barriers are only useful for the first capture point, the other two don't really benefit because of the layout, but extra troops are always useful and were instrumental in my fight.
Specifically I left something like four Khorne Warriors w/shields and four pink horrors to hold the line against the endless spawns from the bottom, backed by towers and barriers. Everything else, including most of the reinforcement units I could spawn fought through to the second capture point, then on to the third for the final showdown. I wasn't well equipped to kill Soul Grinders though, so they were a slog to get through each time they showed up.