I use either a large Stillson plumber's pipe wrench or a large adjustable wrench with a piece of pipe on the end of the handle to give more leverage.
 To hold the tank .... you could ratchet strap it down or use rope with a trucker's knot.
 I have used a Sawzall with a coarse metal cutting blade before. Just cut the brass valve off & don't worry about the little chunk of brass left in the threads of the tank.
 In theory: You're cutting brass so it won't strike a spark. 
In reality: Maybe not the best idea.
 
 ETA:
Be careful with the newer type valves ! They're deceptive because with the valve fully opened they won't let any propane out. ( It's a safety feature )
 If you're working on a propane tank that's still under pressure it's running at about 200psi. Pull the valve off that one and it's a bad surprise.
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