Lions that could survive the drought were those that adapted to hunt elephants
No. I never said that. IIRC while the pride that was learned & went on to prefer attack elephants prospered, other prides in the area suffered through the drought, but survived.
if you wipe out all your prey in your area
Lions weren't the only reason or even the most important one in the extinction of elephants in the pride's area. Humans reduced the elephant population to the point that they could be locally exterminated.
Pack animals predating much larger prey isn't a freak event. It has happened throughout time & all over the earth whenever a prey species has evolved to be much larger than the available predators. There may be a window in which the prey species is invulnerable but eventually pack animals will adapt to the available resource.
If 29 meter (terrestrial) carnivores ever existed, we would have seen some sign of them by now