Corporations don't have to be evil - they choose to be in the service of profit. They don't have to be monopolies. They don't have to be giant sized.
A 100% tax on all revenue and income over $100 million a year will keep growth down and make corporations smaller. They would have to focus on their local and regional customers. There would have to be multiple corporations doing the same things to fill the market demand. So they'd have to compete.
It will also hopefully encourage the formation of more co-ops, since they won't have to compete with giant monopolies anymore.