Artifically low prices, endless and expensive litigation (unless you count that as government siccing), corporate espionage, bribery, acquisition, extortion, etc.

Is it really that hard to think of what you could do with a coporate money and know government intervention?

If your answer the question of how being a billionaire leads to monopoly is 'crime', I'll have you know that that can be done independent of billionaire status.

A billion ways.

Here's just one: run at a loss until your competition is driven out of business. Then raise prices once you're a monopoly again. (This isn't a hypothetical either, but a standard practice.)

This doesn't work. You can cut prices and lose money until the upstart firms go out of business, but you cannot raise them afterwards or guess what will happen?

Potential upstarts can't get funding to start their business, because investors know you'll just bully them back out of the market.

Unless they find another billionaire with pockets as deep as yours, and a willingness to wage a price war for years.

You'll become Walmart?

Walmart has low prices on everything. If they are charging monopoly prices, why is their margin 3%?

Buying up all the land that contains a particular resource then guarding it, thus permanently locking said resource away is one way to monopolize a market vertical.

If there had ever been such a person, even once, it seems like we'd be celebrating them today. If you could send a guy back 200 years to keep all the coal in the ground, you'd do it.

There are very few that will celebrate De Beers implementation of this monopoly business model: https://en.wikipedia.org/wiki/De_Beers#Legal_issues

De Beers does not have a monopoly. Look up their market share.

They had an effective monopoly, until their control of supply was broken: http://www.paulzimnisky.com/a-brief-history-of-de-beers

By market forces. Competition, when permitted to exist, produces net benefits.

What does this have to do with billionaires? Everyone does from private citizens to governments do this, and land itself has value whether there are resources are not.

Can you give me one example of a billionaire doing this?

They can finance the dumping of product at below market rates until they run the competition out of business, then corner the market for themselves.

You cannot corner a market through price dumping. You can lose money until the newer firm dies, but once you raise prices again, guess what happens? The Baltimore Gas Company is a famous example of this. They didn't establish their monopoly until the maryland legislature enforced a utility charter that granted them the exclusive right to distribute gas in Baltimore.