Whoops, after reading Starfish's post and rereading the question, I think I miscomprehended what you were asking, sorry
Starfish has it correct. I'll expand on the double-killing bosses bit: apparently, there is a bug in Disgaea 4 where you can infinitely kill item bosses to get the boss-kill bonus, as long as you have Gency's. In previous Disgaea games, you would only get the boss-kill bonus if you cleared a level and also killed a boss, leading to "only" getting 2 boss-kill bonuses per boss.
Not sure if you wanted to know this information as well, but character stat growth is based on lvl 1 base stats (stats without equipment). When you reincarnate, your character's lvl 1 stats are based on the base stats prior to reincarnation. It is possible to make your lvl 1 base stats go down if you repeatedly reincarnate at a lower level than the last time you reincarnated (i.e. reincarnate at lvl 500, then lvl 400, etc.), so, to avoid suboptimal stats, always reincarnate at an equal or higher level than the last time you reincarnated.