There are a total of 21 Chapters in the main story, and since you need to beat the Campaign 4 times that gives you 84, but you'll still need 16 more.
So the Quickest Chapters to beat would be
Episode 3 - Chatper 1 and Episode 3 - Chapter 2.
On Episode 3 - Chapter 1 you have to defend the train which can take 10 - 15 mins depending on difficulty.
On Episode 3 - Chapter 2 you have to kill some attacking enemies and then two boss Akrid.
Honestly I prefer Episode 3 - Chapter 2 its less boring and you can work on the damage Nom's and Killing 30 Bosses.