theres really no fantastic place to level up. The black omen is the best but the way to do it is go back through time each new game. For example, Beat the Black Omen in 1000 AD up until right before the mommon machine and Zeal, go all the way back through fighting any monsters still there along the way. Go back to 600 AD do the same, then 12,000 BC and beat Zeal. I've done it this way and its great for leveling up because if you kill things in the future (like the mini bosses along the way in trhe black omen) their still there in the future.

I haven't tried it this way, but theoretically, you could kill zeal and the mammon machine in each time zone as well, because theres a gate just as you step inside Lavos shell, and you could keep going through the whole black omen. But I've never tried it this way.

Honestly its really not worth even getting to lvl 99 though except for to fight spekkio or really just for the sake of getting to level 99. Lavos is so easy in the higher leveals that hes a joke