because the lich took over everyone and told them to kill her

Lich? take over everyone? *bitch please* finn and jake could kick his ass.
why is Marshall lee and Prince gumball up there but not Fionna and Cake?

Because they are the ones fit to be the Post-Apocalyptic Assassins. (Finnzio and Jakelonardo! :D)
But, Marshall "Lee"?
AGH ITS THE DESCENDANT OF THE ONE WHO KILLED CONNOR'S TRIBE!
(Charles Lee, if you ask)
OFF WITH YOUR IMMORTALITY!

^ "OFF WITH YOUR IMMORTALITY" XD that's a good quote, you make it? :D

@Finnzio Mertens da Ooo
Yep :D

OFF WITH YOUR **IT XD

^
lol wat

Bullshit from Uncyclopedia is better, THEY HAVE SHIT MAKING ROBOTS :D