There's enough showing there to grip with mole grips and then brute force it out.
Failing that, cut the head off, remove chassis brace, then clamp mole grips to it whilst it's entirely off the rig.
It's very rare I experience this with my rigs, but funnily enough today I was fitting a rotary washing line for someone and one of the cross head bolts clamping the tube going into the ground decided to play silly buggers.
I cut the head off and removed it that way.
I've heard people say that the screws on Corally rigs are soft.
Personally, I think the main issue is that RTR screws are tight with thread lock from the factory and the majority of users end up using the crappy L-shaped allen drivers provided with stuff, which aren't precisely made and quite soft. They readily chew up hex heads - it's a one shot deal I'm afraid.

Or:
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