Anyone know how Luffy got he scar on his chest? I can't bring myself to remember -.-

That would be Akainu. During their attempts to escape, after Luffy had fallen unconscious following Ace's death, Jimbei was doing his damnedest to evade the Marine Admiral. A savage punch, penetrating Jimbei's body and ultimately hitting Luffy in the chest, was the cause of the scar.

akinui one of the 3 generals os the marines put his lava arm ou something through ace and luffy but ace died and luffy was in a critical state

It was originally a hole made par akainu but law healed luffy and that's why it turned into an x :D