it appears each LOGIA user has an achilles heel, a counter-fruit out there on the grand line. some are obvious: ENEL(lightning)/rubber, AOKIJI(ice)/fire, CROCODILE(sand)/water. but in contravension with this theme, oda allowed a logia-user to be defeated by something OTHER than his "natural enemy." i mean, i'm not sure WHAT fire's natural enemy would be … maybe WIND/WATER ... which are to see (!!though everyone here on AP has their suspicions as to WHO ate those fruits!!). but LAVA, come on, seriously :(
PS:
WIND = Dragon (duh, go watch loguetown again if you're stupid enough to miss that one)
WATER = Roger (maybe >_> can you think of a more powerful devil fruit ... only weakness i can think of is ICE, and it wouldn't kill you, just imprison you, and you'd be immune to seastone O_O)