Disclaimer - Difficulity applies to those with a higher rank. It is always difficult for low ranks.
Bosses in R2DA are rated from 1-10 in terms of difficulty, depending on strength, attacks and general superiority of weapons during the time in which the boss was played. The same boss may have different ratings due to various factors.
A corresponding difficulty comment is associated with each number. There is a brief description on what each difficult comment means at the bottom of this page.