Fill plug on TC

I have learned a valuable lesson on the difference between 12 point & six point sockets. When purchasing a 30mm socket to remove my TC plugs (np231), I asked the the salesman if they had a six point in stock "we only have the 12 point, but don't worry it'll do the same job..."
The 12 point socket "did a job" rounding off the corners on my tc fill plug. I have since used a pipe wrench and vice grips with no luck.
I'm probably going to drill into the plug to extract it unless anyone else has a better idea. BTW: I do not own a welder to tack a bolt to the plug.

Chisel and hammer. Catch the chisel on what's left of one of the points and give 'er a whack, I've pulled MANY stripped plugs this way.

John