FFVII's EXP system means that each new level requires more EXP than the previous one... so getting from level 98 to 99 is the biggest jump there is. I'm not mathematographist, but judging from the small number of EXP you earn from defeating the monsters and guards in Midgar, I'd guess you'd have to slaughter perhaps half a million enemies to get all the way to level 99.

Two options, basically: either use a cheat device such as Gameshark, or waste many hundreds of hours of your life that could be better spent on... well, everything else that FFVII provides for you to waste your time on.