Full pressure - good luck, it's expensive! You'll need a custom made ram or modified hydraulic cylinder and to properly build and pressure test your system - off the shelf components won't do for a lot of it. Terrorhurtz doesn't use full pressure and I'm sure you know just how powerful that is, so you really don't need full pressure to make a powerful axe

Also, just a heads up - a powder fire extinguisher is pressure tested to like 20-30 bar, working pressure is more like 10 bar... not the kind of thing you want for a full pressure buffer tank! You really don't want to scrimp on pneumatic components, 150psi is scary enough when things go wrong... 750 would be positively terrifying.

I'd advise sticking with a low pressure system - you can get rams, valves and such like far easier, from surplus suppliers and ebay if necessary. It's also slightly safer, especially when you have little experience with pneumatic systems - though that's a relative term, these things can (and have) killed people... I'd STRONGLY advise you to get some help with setting everything up from someone with pneumatics experience.

As for what you'll need: CO2 tank (fire extinguisher will do it), CO2 bottle valve (comes with the fire extinguisher - you can modify the valve on it by drilling a hole through the handles so you can stick the pin through to lock them in place with the valve open), regulator or regulators (you may want 2 in parallel for higher flow rates and better tolerance of icing up) to drop the 750psi down to ~150, buffer tank or tanks, pressure relief valve set to ~165psi for a 150psi system, dump valve (to let the gas stored downstream of the regulator out when shutting down the robot after closing the main CO2 bottle valve) 5/2 solenoid valve, probably a high flow solenoid valve in parallel with the extend half of the 5/2 to increase your flow rate while extending the cylinder (means your weapon will move faster), quick exhaust valve on the rod end of the cylinder (exhausts gas from the other side of the piston faster when the piston is extending - again, quicker weapon activation).

That's just about an effective system, but you'll also need the axe mechanism - I'd suggest a rack and pinion setup similar to what Terrorhurtz uses (you can see a diagram on their website) as it's small, light, reliable, and tough - you will need to get some parts at least made in a machine shop.