• A lawyer representing laid-off Twitter staff sent a fiery letter to Elon Musk's lawyer, Alex Spiro.

• Akiva Cohen claimed his clients weren't receiving the severance package they had been promised.

• Cohen tweeted that he hoped Musk would do the right thing, otherwise "it'll be fun as hell."

A lawyer for fired Twitter staff has given Elon Musk a deadline of December 7 to confirm that he will pay them full severance as promised, or face an arbitration campaign to settle the dispute.

Akiva Cohen — a partner at law firm Kamerman, Uncyk, Soniker & Klein — tweeted a copy of his letter which addresses Musk as the "Chief Twit."

He accused the world's richest person of "attempting to tap-dance your way out of Twitter's binding obligations to its employees."

"If you don't unequivocally confirm by Wednesday, December 7 that you intend to provide our clients with the full severance Twitter promised them, we will commence an arbitration campaign on their behalf," Cohen said.

Since Musk took over the company, Twitter's workforce has fallen from 7,500 to 2,300, per Insider's Kali Hays.

That means almost 70% of staff were laid-off, mostly during the first round when Musk halved employee numbers, and his ultimatum to commit to working "extremely hardcore" or be laid-off with three months severance.

The cuts resulted in the closure of Twitter's office responsible for complying with European misinformation laws, and just one employee left on the Asia child safety team.

One executive has also been reinstated after she was dismissed for not responding to the "hardcore" ultimatum.

Musk now stands accused by some former Twitter staff of failing to provide the severance package they were promised, as alleged in a previous lawsuit.

Cohen said that his clients weren't receiving their full benefits, like 401k deductions.

His fiery letter says: "To be clear, Elon, you will lose, and you know it."

He adds that even if Musk did win, it would be "Pyrrhic" because "Twitter will pay far more in attorneys' fees and arbitration costs than it could possibly 'save' in severance due our clients."