You could stretch the line out longer if you space out the heavy hitters with some of the cooler 1 shot villains and by doing all the Justice Ducks
I'm thinking something like
>wave 1: DW, Gosalyn, Taurus Bulba, Megavolt
>wave 2: Launchpad, Liquidator, Stegmutt, Steelbeak
>wave 3: Negaduck, Splatter Phoenix, J Gander Hooter, Bushroot
>wave 4: Gizmoduck, Quackerjack, Bugmaster, Grizzlikoff
>wave 5: Dark Warrior Duck, Neptunia, Honker Muddlefoot, Camille the Chameleon
>wave 6: Drake Mallard, Morgana, FOWL Eggman, Dr. Fossil
>wave 7: Paddywhack, Jambalaya Jake, Tuskernini, Herb Muddlefoot
And by this point you probably run out of steam unless there's a market for stuff like Rubber Chicken