What overpowered monster? Every mob at your own rank is super easy to kill.
It may be tough to kill Mobs with a slightly higher level in the beginning when your physical level is low but later? I easily can rank up a job 1-20 r in just a couple hours by killing mobs way above my rank with SP 300 – almost 500 per kill. Easier even if you make some leves.
No offense but you might simply want to practice your skills a bit more.
If a agro mob manages to hit you 2-3 times before you can draw your weapon and lock on then your reaction is simply very poor.
Get some healing abilities!
Learn how to avoid Mob special attacks! (most you can dodge by simply walking around the mob or running out of range)