Assemble a New Team on Beginner Difficulty.
Characters like Hisako, Riptor, Jago & Tusk all have very good normal movesets that work really well against the odd AI. But play with who you know best.
You only have to REACH Turn 20- so technically you only have to complete 19 Turns. It should unlock right at the beginning of the turn.
On this difficulty the AI hardly ever Combo Breaks, so generally go for Heavy Auto Doubles & Openers. The Ram or Poison Bite Guardians worked best for me.
Past level 15, you will have to really think about whether or not to deploy.
The easiest thing is to do the "controlled corruption" method that everyone uses where you specifically skip missions in only one (or two) areas. This way you don't all of the sudden have 4 omens on one turn, and it becomes easier to control when you want your corruption to reach the 50% and 100% marks.