Zaku III is an absolute monster at 550-600. As you can see from this video it can easily stop suits such as Dreissen with either its beam rifle or head beam gun. After which it can do a triple downswing combo for huge damage or combo to cannons to restun and do a good chunk from any range. Your head beam gun actually stuns in only 1 second if every shot hits and can be used while abusing your MA by boosting and shooting...
It is easily an S tier suit and well worth learning if you have one.
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