Defense strategy
When playing as any character with a defense upgrade (besides Aellita), it is easy to face any monster. When the monsters appear, hold your shield and watch their attack pattern. When the monster you want to hit is going to shoot you, release and press B. When the bullet hits, it will rebound. If the monster does not react, you may need to get closer. Continue doing this until all monsters die. Note: This does not work with any Boss.