If you want to gain Monocoins as quickly as possible, I recommend using the Raise, Reraise, and Super Raise skills. If you want to maximize the amount of experience that you get, you should also equip the One Nighter, Daily Effort, and Super Studying Technique skills.
In order to use this method, you need to reach the boss battle in Chapter 3. You will want to keep the battle at its initial stage where two normal Monokumas are summoned into the arena at a time.

For this strategy, you'll want to avoid the boss entirely. Don't attack the boss so that you can keep the battle in its initial stage where only normal Monokumas appear. The boss has two different types of attacks which are relatively easy to avoid.

• Mega Attack and Mega Piston - The boss will swing its axe into the ground once and will be immobile afterwards. To avoid the attack, simply run from the boss when you see it wind up for a swing.
  
• Mega Splash - The boss will swing its axe three times and will be immobile after the third swing. In order to avoid this attack, I recommend running along the edges of the arena until the boss swings for the third time and is immobile.

After the boss attacks, its axe will be stuck in the ground and it will be immobile for a short period. You should use this time to kill the Monokumas that are summoned in the arena. If you shoot the Monokumas in the red eye, they will drop Monocoins.

If you are low on health, hearts will be spawned into the arena. Ammo will also spawn into the arena or be dropped by Monokumas that are killed by the boss. Repeat the process of avoiding the boss and killing the Monokumas until you are satisfied with the amount of coins and experience that you've gained.