Yeah you did the same mistake I did thinking you can just play like part 1 & 2 and advance chillax, but no, some of the bosses in Very EASY would one shot my luka despite having over 3M attack in stat, the answer to that is both very easy and potentially annoying depending if you like grinding or not. Basically find an item or ability that boosts your "job" xp and then level a few or all the jobs/races you have unlocked on sealed and forbidden to max, the max gives you per JOB an ability that cost nothing you can activate and it boosts the character (in my case Luka) considerably. The other option is to make sure you find and recruit every blacksmith dude, the more you have to more weapons and armor you unlock to craft and to be honest they are brutally overpowered, so get them tiger. (you can also just level the characters, but to be honest the stat increase that way is laughable)
Here is an example of the maxed out job/race ability that I mean they cost 0 so you can activate all of them if you have them, and see the boost PER is pretty damn good, so stacked it becomes insane.
With all of this and some of the best gear (no Labyrinth) my Luka now has 162.755.863.053 Attack Stat Which is 162Billion I guess, I can tickle the Last boss and one shot it. This is all without cheating, I do believe TrTr did go bonkers on the numbers x)