Got this tip from a random person.
Farm 1 Command Point and high EXP points in 1 battle:
Complete all the stars in chapter 2 and play the Bullseye mission (chapter 2.3). Play all the missions including Hero Deploys (Black Cat needed), Low-level villains, bosses and wait for the Epic Battle vs Elektra to appear.
Hit Elektra until she drops 1 Command Point, then intentionally lose the game. You still get high EXP points even if you lose. Repeat.
I get 65 EXP per round with Elektra, My current level is 132. Not sure if the 65 is a fixed amount or will the experience given depends on your current level.