Need help or tips from experienced Disgaea players.

Just started playing this again PSP (the first one). Last time was on the DS but got tired of the grinding and quit after a 100 hours and never beat it. One of the biggest reasons was because my entire army sucked except for Laharl. I think the highest damage i reached was 10k or 20k but i see this videos everywhere and people reaching trillions of damage (literally). Wtf how you people do this? And the most curious thing is the level when they do this insane damage. Level 1...

So what im saying is that i suck at disgaea and i need tips. I think i reincarnated laharl like 5 times but i see people doing it over 100-1000 times.