Air hammer on the end while trying to twist it. Need hammer time while twisting like Samwhich said. If you don't have an air hammer...id say blow torch if you are good with one. If not you will probably mess the powder coat up.
Could maybe find a long thick punch/pry bar that tapers down small enough to fit into the whole. Something like this...
https://www.google.com/search?rlz=1C1GCEV_en&biw=1707&bih=846&tbm=shop&ei=huFVXc7bEqKp_Qb85rqwCg&q=long+punch+pry+bar&oq=long+punch+pry+bar&gs_l=psy-ab.3...5242.9740.0.9879.0.0.0.0.0.0.0.0..0.0....0. ..1c.1.64.psy-ab..0.0.0....0.4a5tEIR8g2k
Then put a pipe over it and use some torque.
