Wu wei. He waits and the problem solves itself.
Poison pill tries to increase the market cap of the company, so the hostile buyer can't afford them anymore. However, this is only an effective strategy if the company is chronically undervalued. (Twitter is chronically overvalued.)
Step 1: Musk triggers poison pill. Step 2: institutional investors pour money into Twitter, Twitter debases their stocks so they can hand new ones out in exchange for this money. Step 3: Musk sells at new inflated price, retracts offer. Step 4: Musk waits for $TWTR to go back down to its equilibrium price; institutional investors lose their shirt. Step 5: Musk offers $43 billion again.
They can poison pill again and lose even more money, or they can accept his offer. Repeat as necessary.
Unless Blackrock is in fact Fed-backed, of course, and doesn't give a shit about how much money it loses. It can dump cash into that hole endlessly, because it's not their own money. Of course America is already risking hyperinflation, so there's that.