TOPIC:

I need to strip out the shaft that drives the waterpump, but I can not get the bolt loose on the plastic gear side. I have tried a good couple of times, but I am scared to strip the shaft that is suppose to take a 12mm wrench. I read the manual and hoped that I would get more info like left hand thread or something but no luck.
Can somebody please give me a bit of advice on how to loosen the bolt without damaging something else. I have attached pics for some reference.
Thank you in advance.

I see the hex parts on the shafts are already very much stripped, so it will be hard to keep them from turning.
I think the mistake you've made is using a very thin wrench to hold them - very little contact surface. Now, the best is to use as wide as possible wrench, or even better two if they fit and there is enough undamaged surface to secure them. That's what I do - two wrenches. Also, a piece of wood between them and the crankcase, so the case will not get scored plus it will help choosing perhaps a better angle for the wrenches. I'd also use an impact gun, and these threads are not left but regular.
Given you can hold the shaft from turning, the bolt should let go. If it doesn't, it will become more of a real rescue mission than anything else.

Will this type of spanner work to hold the shaft?

If it fits and the shaft is not rounded too far why not. But I would use it with one more wrench, or as many as you can fit. The idea is to have the whole length of the hex area on the shaft held by something, so the tensions will spread instead of being in one place. This will prevent the hex profile from damage - once stripped you can't hold it anymore. If you have regular wrenches, and they are solid not sloppy so they fit tight, just use 2 or 3 if they fit. Turn the bolt until they jam against the wood so you don't need to hold them, perhaps jam a small piece of wood on the sides as well so they want tip, and undue the bolt with an air gun - turning left to remove it.

So I followed instructions step by step, and i eventually got it out. Unfortunately the plastic gear broke so i need to get a new one.

The trick that did the job was that i tightened it first before it released.

Thanks for the advice, much appreciated.

Well done! It often does help to tighten fasteners before trying to undo them, especially if corroded. It seems to break the bond.
Hoping you keep us posted on the rebuild.