How can you beet Chaos Grimmon in Digimon World Dawn

You don't actually need four megas. You need digimon that are able to take and give hits without expending too much MP, preferably attacks that hit at least 2 zones. In dawn, Apollomon is good, and any other Mega/Ultimate digimon. If you can beat the first stage, he will give you enough EXP to level up several times, and even if you lose the second stage, you could probably evolve your Ultimates by then.