Lemme give you the short version: you can get Tonberry King at any level. EXP level affects absolutely jack in this game as long as you abuse the oh-so-broken junction system ^____^ Now go drop a train on 'em!

Incidentally, if your level is low but your HP are high due to junction, you'll be barely affected by "Everyone's Grudge" as it relates to your kill count, and low levels mean a low kill count.