Complex sentence: Once the player has knocked all of the enemies off the screen, they advance to the next level, this can prove difficult for the player as increasingly intricate obstacles are presented for them to overcome to destroy all of the monsters and ghosts.
Split into simpler sentences: Once the player has knocked all of the enemies off the screen, he advances to the next level. Advancement can prove difficult for the player as increasingly intricate obstacles are presented to overcome all of the monsters and ghosts.